I wrote these instructions weeks ago when you still couldn't configure the APN from the GUI, I just realised that you can configure it now, so don't mix the two methods, keep the one that works.
We are talking about Manjaro Plasma Mobile. The important thing is that the internet is working. Before some days I configured it with the commands and it works fine every time it reboots. Try to delete everything from the GUI and create only one context with the commands. There may be some automatic setting enabled that I am not aware of.
All the commands are at the end of the thread.
for internet
qdbus --system org.ofono /quectelqmi_0 org.ofono.ConnectionManager.AddContext internet
for mms
qdbus --system org.ofono /quectelqmi_0 org.ofono.ConnectionManager.AddContext mms
Something must be wrong because I have tried it many times and it always works. After you configure the DBus the settings stay there. It is possible to be a difference in the behaviour of different devices but this one is a simple thing. After flashing the image you should have one internet context which should be configured and turning on-off the mobile data 2 times from the menu should activate the connection.
The internet should work for anyone whose device is accepted by the operator and makes calls, now you can either make a script and run it after every reboot or use Manjaro Phosh (maybe temporarily from an SD card) until you find a solution.