I was experiencing quite a strange behaviour with the MG21. I put a multi-vendor sim card in it and allowed all providers. I also had the same SIM card in a MG41.
I have a provider tower like 300m away from me (Provider 1) and like 3km away another one (Provider 2)
Of course Provider 1 had way better coverage and the MG41 immediately joined up on Provider 1 without any issues, meanwhile the MG21 joined on Provider 2 and it was connected to it for 5-6 hours. After this time period the MG21 decided to join on Provider 1.
I've tried with different devices and factory reseted etc, results was always similar.
What could be the reason? Does MG21 and MG41 has any differences in some coverage calculation? or maybe they prioritize different bands?
Also note that carrier cell sites are not "static". In my country (for example), most of the carriers use things like dynamic spectrum management. As cell sites get busy they get allocated more spectrum (which gets removed from less busy cell sites), so the capacity of individual cell sites vary by time of day.
So that cell site very close to you - if it is not that busy you may find that at peak times channel capacity gets removed from it and added to other cell sites, and then later gets put back again.
And that is just one of many parameters that gets managed dynamically. It's a little bit like WiFi in that regard.