So you are on the most recent weekly. This means that safety net passes. Have you tested the safety net to ensure any damage to you ROM is not affecting it? See attached screenshot to show this ROM passes.
If yours doesn't, then you need to format your phone. As I said, if you are going to sell it anyway, what have you got to loose.
Back up your data but do not restore any untill you can see that the Coop bank app and GPay works.
Download the most recent ROM from links here...
https://androidfilehost.com/?w=files&flid=322708 or from this forums link here...
https://xiaomi.eu/community/threads/21-2-3-4.59832/ and save it to your phone or otg usb stick.
Reboot to twrp, install downloaded ROM, then instead of just wiping, go to wipe menu, format data and confirm by typing yes. If you don't get to a confirmation question, you are not in the right menu. You must format, not just wipe.
Now go through phone set up and once done, just download the safety net tester to check again that it is working, then only download the coop app and GPay app, nothing else. You can now check that they work before downloading the other apps. If they do work at this stage, be careful if restoring data as something in it may be limiting these apps. I would be very surprised if they don't work now. I know it's a pain but you have nothing to lose other than half hour of your life. Let us know how you get on.