I believe there were issues with 4G dongle they sell not being compatible with US 4G carriers.

I have xtrons PT1853B which is very similar design to yours but with Android 8, Octa core, 4GB ram and 32GB rom. Installed Resler iBus with DSP license, running 6ft antenna adapter and power harness from back of the car.

It runs good most of times, but once in a while, iBus app won't recognize the USB interface and won't connect.

For your issue, loosing Resler when using Spotify, how are you streaming to the headunit? Enable iPhone wifi tethering and headunit connects to the iPhone, and run the Spotify app from headunit?