Hav u tried to restart ur Phone? Turn Off wifi wen ur out n about bcuz sumtimz its too busy trying to access wifi along the way that it blocks out cellular signal. If all else fails contact Apple Tech Support or Talk to support with ur phone carrier. Depending on ur carrier it cud b a cellular service issue. Process of elimination first.
Restart
Turn off Wifi
Cellular Provider Support
Apple Tech Support (most of the time u hav to purchase extra support service contract with Apple which is insanely expensive which is y i wont do Apple products. Without support contract or paying for support ur screwed n end up havn to file an insurance claim n send the phone in then ur without a phone til theyre dun. I stick to Android Products and wont even buy and Apple Products.)
just an fyi. u hav to follow the steps above in that order bcuz they will ask.

Yeah sounds about right remember I got lost with the HivTeam site they created a new username and I had to start over 😭the good news I didn't loose my friends 🤗

If you're not comfortable with doing it yourself and don't want to mess up anything, let the phone store do it for you. Hope this helps.

Make sure mobile data toggle is on, and prefered network is set to global. Roaming up to you. If you get charged for roaming, leave it off, if you don't leave it on.

I think go to settings and turn on Mobile Calls not Data but then I'm not sure 🙃