I think the problem is with the modem they used in Android Pie: it supports only B1 (2100), B3 (1800+), B5 (850), B40 (TDD 2300) and B41 (TDD 2500).
So 4G is working but only in countries where this bands are used. In Italy for example we have band 3 in highways and cities, but in rural areas we use band 20 and it's gone
I hope that they can solve the problem with a new update.
PS: Don't try to flash 8.1 modem over Android Pie stock rom.
It will cause system crashes and SIM card won't work anymore until a reflash of Pie modem.