This for me is the thing that doesn’t quite fit in the picture.
Do you get any indication of G or E or H or LTE with the full bars?
If not, perhaps the full bars icon is wrong and you should have a look at the antenna connection and the contacts …
I remember back with LineageOS 14.1 on rare occasions I lost all mobile network connection by simply leaving the range of my home WiFi … and the status icon would show me full bars without any indication of G or E or H or LTE, which should mean a mobile network connection with plain 2G data speed, but there was no connection at all.
Turning flight mode on and off or a reboot fixed these, and this didn’t happen for a long time now, so probably your case is different, but you never know …