I believe you already know how to flash with EU ROM. It was stated on the first page of this post.Can i please get a link to where is the official thread with installation steps and getting the wallet to work and update on the APK that fixes it?
This is the 3rd or so time i chase the issue and stumble upon a resopnse with a solution.
You need to download the latest ROM and install it on your phone. Then, install the latest EU APK (DEC 05) https://sourceforge.net/projects/xiaomi-eu-multilang-miui-roms/files/xiaomi.eu/Xiaomi.eu-app/
After these two steps, your phone should be able to use Google Pay.
If you want to root your phone, do the above two steps. Then root your phone. Then add goolge pay, GMS, and play store to the Denylist and turn on "Enforce Denylist". Restart your phone. Google pay should works.
If you are beginner, let us know which step you are not clear.