I am not an Android user anymore, but on Android the the Beta and the regular version are rolled into one stream of updates in the Play Store. So once the Beta version you have is too high, even if you leave the Beta program you need to wait for the next regular production update until you get put back on the regular track.
You can download the APK installer for the current version here:
That should get you going. However, remember that installing an APK means that you won’t get Play Store updates for this app anymore. So do this only for now, and when the next regular update arrives, reinstall it from the Play Store. That should return everything to normal including updates from the Play Store