Thank you guys, so this is REALLY strange. I’m really trying hard to understand this phenomenon, but it’s beyond me. I’ve tested it with Android 11, 12 and 13, no change, always MAJOR problems with connecting to 2G network when only one SIM is inserted, whereas 3G and 4G connection is instant. The most bizarre thing is that the problem with 2G registration instantly vanishes as soon as the second SIM is inserted (and what’s the most strange is that it can be even an old and inactive SIM, and I can even disable it in settings).
One remark: that second SIM has to have PIN code either disabled or the code needs to be entered, or else it won’t work.
So far I was thinking that it was just my device’s peculiarity, but @domo141’s description looks identical, so it’s clearly the same case.
It would be great if @domo141 could test and let know if inserting the second SIM fixes the problem also for him, as in such case we might try to find some similarities causing such strange behavior of our devices…
Anyway, if everything else fails, I think I’ll send my 10 III to Sony service centre as it’s still under warranty. So I guess it’s up to them to make it work with 2G network with only one SIM, no matter if it is a hardware or software issue…