Hey again!
Well you have a backup, so you’re ahead of the game already!
I would boot to recovery mode as I think you did:
Startup holding Command R.
The first thing I’d check is the date and time the system is showing:
On the top menu, select Utilities > Terminal. Once there, type “date” (without quotes) and hit Return, the results should reflect the current time and date, (It May use universal time (UTC), so be sure to check current UTC time online), if that’s all good, you can hit Terminal > Quit Terminal.
Then in Disk Utility select three startup drive, (Usually Macintosh HD).
Then on the top menu, select View > Show All Devices.
Then above the Macintosh HD’s you should see the Container Disk, and above that, the SSD/HDD.
Click on the main drive, (SSD/HDD) > Erase, make sure the format is set to APFS, and scheme to GUID, give the volume a name (Usually Macintosh HD, it may default to Untitled, but you can change the name to your preference) > Erase.
Once done, Quit Disk Utility > Reinstall Mac OS.
Make sure you’re connected to your network first, otherwise you may get a server-type error, but you know the drill.
It sounds to me like the recovery version and the current OS version maybe different, so this is a good way to reset it.
Once done installing, (If you need to), you can select the option to move your data back from your backup drive, (Or skip that and set it up as new, you can still access the backup files later if you need to).
Hope that works!
