I used both models for a long while. I had no problems for a long time. Then it started with missed calls while the phone was turned on and had connection. After restart it worked again for a while. Then it stopped completely. I wanted to switch back from fp6 to fp5, but it does not work either. Putting the e-sim on a samsung phone and it works fine. Putting it on either fp5 or fp6, I have perfect data connection but can neither place or receive calls. It onlys tells the number is not available. If I try to make a call, I only met with silence.
Which country which provider? Did you try to disable VoLTE?