I have been coding a MQTT based app for the RAK4631 on a dual I/O baseboard, using the wisblock BG77 modem. I was experiencing intermittent cellular operation which showed up during the QMTOPEN (open a TCP connection to a MQTT broker) statement. I discovered in the quectel mqtt application note that the BG77 can either operate cellular data or it can perform gps. There is an config option to help. Afterwards I removed the BG77 gps power enabling I/O and my MQTT app started working reliably. It appears that if you want mqtt to work, you will need to turn the GPS off - or never turn it on in the first place.
It is well known that newer Quectel chips like the BG77 or BG95 need to switch off GNSS to use cellular connection and to disconnect the cellular to use GNSS.
It is a limitation implemented by Quectel.
I’m afraid that the limitation, and the consequences of it, are not well highlighted within the wisblock ecosystem. Even within the Quectel specs it’s not presented very well as a significant limitation. For me it pretty much eliminates the GPS portion of the BG77 and I have added an external GPS.