I have downgraded from Android 12 to Android 11, due to the fact that the new Android 12 has bad battery lifespan and the OS was very buggy (system will often hang and have to force to reboot manually). Will wait for more stable Android 12 then.
Using Android 11 (Oxygen OS 11.2.10.10.LE25AA) with Singtel 5G has no problem. The small trick is to put the sim card in slot 2 first, and then put back in slot 1.
I am not sure whether Singtel 5G will work for this trick using Android 12. Maybe this will work?