I might also be suffering from this problem:
The symptoms are nearly identical to mine, though I am also having problems when running on USB only.
Is there a command or way to run a hardware self-test on the Quectel RM520N-GL modem itself?
I will probably buy the alternate adapter board that iamromulan recommended and transfer the modem across, unless the modem fails its self-test, if such a test exists.