No sim after upgrade to ios 18.7 / 26.0.1 on iPhone 13 mini

Go to Settings/General/About and look at Modem Firmware. Is the entry blank? If it is you need to reinstall it by restoring the phone to factory settings, then restoring a backup. Back up the phone first, of course:

1. How to back up your iPhone, iPad, and iPod touch - Apple Support
2. Restore your iPhone, iPad, or iPod to factory settings - Apple Support - follow the instructions to restore using your computer, do not using Settings/General/Reset (which is not included in those instructions)
3. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If the modem firmware DOES have a value you may have a hardware problem. The first step is what I just posted, but after that you need to contact Apple support using the Get Support link at the top of this page.

This is a immediate help needed! Go on CELLULAR DATA on settings, and check if your cellular is working! Then check you information if they are correct or valid! If they are not, contact Apple VERY IMMEDIATELY! And call your service for more info!

You have Mobile because you are outside USA probably UK?

You cannot go to iOS18

Your modem hardware possibly failed during the update. Have your phone analysed at Apple Store

Downgrading won’t fix it. Contacting your carrier might.

PS I am in UK and have no problems with Spusu, Giffgaff or Lebara which are MVNOs representing the major mobile carriers here