That phone supports LTE but not VoLTE. That means it was 3G only for calls, therefore now without 3G its only means of receiving calls and texts is to fall back to 2G.
There’s nothing that can be done apart putting the 3G towers back.
The second next option is to force it to LTE, where you’ll get only a data connection and you can use VoIP calling apps to make and receive calls and texts.