Manual selection of the network, that's what I meant, unfortunately for you they don't seem to have roaming agreements between operators. In France, Free and Orange have such an agreement, with Roaming disabled on the phone, if Free does not cover the area, the phone automatically switches to Orange and vice versa.
It also allows me to force a connection to Orange if free is overloaded or just to test services.
Manufacturers can make a package by region / country / language or a single package that groups several when the differences are not important. The specific parts are applied during the update.
As the package is developed and tested in English, some behavioral differences may result. it is not really the case for the system, more for the applications but it is worth trying with the tel in English. i just saw it :), i had voice / 4G disabled, when i switched to english i got a notification telling me i should enable it. if I am (the rog5) in English with voice / 4G disabled and I change the language, I do not have this notification.
No, re-install the same firmware but you can also go back to the firmware of your choice, @Danishblunt to post an XDA-devellopeur link which explains how to do this, page 6 on 08/17 of this topic Wifi & Wifi-Hotspot Not Working