No Connection to Carrier from Pinephone 3G Model

[nonoreply]

After I had removed and reinserted the midframe and after the preventive resetting of the U.FL antenna connection, the phone was able to establish a relatively stable internet connection (LTE, less than 35% Signal Strength) for about 30 minutes (but no calls). After that, I have flashed the phone's eMMC from Mobian back to Manjaro/Phosh. Still no calls + highly erratic registration to carrier/provider/cell tower.

So far so bad ... it is really unsatisfying. I am not able to track down the real cause of the modem related issues. Does anyone have an idea how to distinguish between bad/broken modem or bad/broken signal reception (antenna)?