That's a very similar symptom I observe often: Apparently, there is a signal, but calls can't get in or out, and finally the phone "gives up" and you get the "no network" message.
Unfortunately I have no solution. For me (German carrier), calls do work in principle, but only with specific cells/bands, and only with VoLTE. Do you have the latter enabled?
At this point, no one really knows what is going on. Sadly, if F(x)tec do not come up with a fix for this, be prepared to never be able to use the Pro1X as a (reliable) phone ...