Had problems using 4G when I transferred my sim from my old mobile (calls, sms, and wifi were all ok, but data not…).
After much searching and trying the solution is to manually set the APN settings:
Settings/Network & Internet/Mobile network/Advanced/Access Point Names
Click on the +. Change to the settings below. Click save.
MMS proxy: 184.108.40.206
MMS port: 8080
Authentication type: CHAP
APN type: *
APN Protocol: IPv4
APN Roaming Protocol: IPv4
Leave all other options blank/Not set