I’m almost sure that signal problems was also before you change the screen it’s just was hard to see it’s because randomly.
That problem very common in ip6 model signal problems the way to check when no signal dial *#06# imei check, if the number didn’t appear the modem have disconnected.
Its can be repair just make sure the technician know what he doing and if he will do it good its will not happen again.
The reason for modem problems is falling and the modem is sensitive sny hit can cause that.