Mac mini 2018 recovery mode issues

A Troubleshooting Procedure that may Fix Problems with macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

    Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Disconnect all third-party peripherals.
3. Resetting your Mac's PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences.
6. Start the computer in Safe Mode. Test in safe mode to see if the problem persists, then restart normally.
7. Use Apple Hardware Test to see if there is any hardware malfunction.
8. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list.  Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
9. Repair permissions on the Home folder: Resolve issues caused by changing the permissions of items in your home folder.
10. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
11. Download and install the OS X El Capitan 10.11.6 Combo Update or 10.12.6 Combo Update or Download macOS High Sierra 10.13.6 Combo Update or Get MacOS Mojave now from the Mac App Store as needed.
12. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
13. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

     1. Select Disk Utility from the Utility Menu and click on the Continue button.

     2. When Disk Utility loads select the drive (out-dented entry) from the Device list.

     3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.

     4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)

     5. Click on the Apply button, then wait for the Done button to activate and click on it.

     6. Quit Disk Utility and return to the Utility Menu.

     7. Select Reinstall OS X and click on the Continue button.

Yes, your warranty should be honored in the EU. If you are unable to boot the machine in any way, then I don't think any of the following will help. Apple has publicly announced that Best Buy is now fully authorized to fix all Apple products using Apple parts and trained technicians. If you can find a Best Buy then perhaps they can fix it for you.

I have limited information about the T2 chip. You can take a look at Upgrading a MacBook Pro 2018 with T2 to Mojave may fail, Firmware, the SMC, NVRAM and T2 chip- reset and maintain – The Eclectic Light Company and Apple changes SMC reset on T2 Macs, and updates NVRAM reset info – The Eclectic Light Company.

I have not found anything as yet about booting from an external drive. Have you tried a Network Restore:

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible back up your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (will be the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

I also have the following but don't know if it will help. I am not responsible for the procedure, so I have not tested it on a T2-equipped machine.

Restore Admin Acct. if your Mac has the T2 Chip

1. Restart your Mac while holding down Command-R. Release the keys when the Apple logo and a 

    progress bar appear. This will boot Recovery Mode.

2, When the macOS Utilities window appears, select Disk Utility and continue.

3. Select your startup disk in the pane to the left and take note of its name. (It is usually named 

    Macintosh HD).

     If the name is greyed out, or if you see the text "Not Mounted", click the Mount button. If 

     requested, select your username and enter your password. Make sure that the startup disk is 

     mounted before continuing.

4. Quit Disk Utility. The macOS Utilities window should appear.

5. Go to the top menu bar (where the Apple logo resides) and click on Utilities. Select Terminal from 

    the drop-down menu.

6. Enter the following command, exactly as shown. Substitute "startup disk" with the actual name of 

    your startup disk.

         rm '/Volumes/startup disk/var/db/.AppleSetupDone'

7. The command line should return with no response. Restart your Mac to launch Setup Assistant, 

    and see Fixing your normal account. If your Mac has FileVault enabled, you'll need to log in after

    the restart.

Fixing your normal account

1. When Setup Assistant appears, progress through the Assistant until you're prompted to transfer 

    data. Choose not to, and continue to the next screen.

2. You'll be prompted to create a new administrator account. Give it a separate name to easily 

    distinguish it from your real account. When done, complete the rest of the Assistant.

3. You'll be automatically logged into the new administrator account. If you wish to restore 

    administrative privileges to your real account, and optionally delete this one, keep reading.

4. If you previously lost administrative privileges by trying to change your account name, reverse any

    such changes. The correct procedure for changing your account name can be found here:

https://support.apple.com/en-us/HT201548

5. Open System Preferences and select Users and Groups. Click the padlock icon in the bottom left 

    and authenticate with your temporary credentials.

6. Select your real account, disable Parental Controls if necessary, and check the box next to "Allow 

    user to administer this computer".

7. Restart your Mac, and log into your real account.

8. Go to Users and Groups in System Preferences, and click on the padlock in the bottom left corner. 

    Authenticate with your own credentials.

9, If desired, delete the temporary administrator account.

Make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. Also, see How to find your nearest Apple Authorized Service Provider (AASP) or Distributor (AAD).

"Your Apple-branded or Beats-branded hardware product (“Product”) is warranted against defects in materials and workmanship for a period of ONE (1) YEAR from the date of original retail purchase (“Warranty Period”) when used in accordance with Apple`s user manuals (refer to www.apple.com/support/country). Under this warranty, you will be able to direct your claims to Apple even in situations where you purchased the Apple Product from a third party.” [Apple One (1) Year Limited Warranty – Accessory - For Apple and Beats Branded Products Only.]

Continued from above:

14. If none of the above helps then see How to Downgrade macOS High Sierra and macOS Reversion- 

      How to Downgrade from High Sierra.

15. If you get here without success then it's time to make an appointment at the Apple Genius Bar for 

      service. If you need to find an Apple Store - Find a Store - Apple.