After the update to OneUI 3 google pay doesn't work and the Playstore says my device is not certified.
Have tried deleting the app data from both playstore and google pay and restarted but nothing works.
It would appear that the problem was at Google's end and they have fixed it in the last 24 hours. A simple shutdown and start up worked for me.
You say... "I was in the Beta program but (think) I'm now on the stable version."
Beta releases are not Certified by Google because of their temporary nature.
To check whether you are on Beta firmware or stable go to... Settings > Software update > Last update.
Any stable update will have been installed from the 3rd December onwards and the version will end in.. TKG