ECM pre-requirements

Please advise what should be configured on the modem for ECM mode to work.
The question is related to EC/EP/EM series.
I’ve got this mode working on EP06 and EC25 without any special effort from my side, others reported successful testing on EC25 (again) and EM12. At the same time I saw reports from EM06 and EC200T owners saying that those modems refused to work in ECM mode. Refused means no traffic to/from the Internet.
In all the cases the mode was properly set (AT+QCFG="usbnet" +QCFG: "usbnet",1), host system successfully obtained the IP address from the modem and the modem itself had successful network registration and, finally, connection was established that is confirmed by the IP address obtained from the carrier:

+CGCONTRDP: 1,5,"xxx.mncXXX.mccXXX.gprs","10.x.x.x",...

Dear Jfrog,
Thanks for your inquiry in Quectel forum.
For different module, if you want to use ECM mode, you need to set the following command:
Please note that different module have different mode by default. For EC25 module, the default is NDIS with AT+QCFG=“usbnet”,0; for EM/EP module the default is MBIM; and only EC200T module is ECM by default. Thanks!

The mode is already set as I already mentioned. Is anything else required for the modem to start passing data?

You just need to make sure the module already register on network. Thanks!

Do you have any idea why that can is insufficient in some cases?
A colleague of mine faced a problem with his EM06 that was definitely registered.

It is hard to say what is the reason, we need to analyze it according to the test log. Thanks!

Same happening here with an EM20-G