It is not, it is related to both major phone service providers in my country. Abroad, everything worked just fine. And just in 4G/5G, however 3G is getting phased out, so if I forced it, I was often unreachable.

I was wondering if I could fix it myself, but I'm not even sure if this is firmware or OS issue. I assume the former, which afaik is not opensource? Not sure.