I've updated some sections on the OpenWrt's Wiki.
4G/3G modem: alternative configuration --> qmi support for all those users who are experiencing problems with C3 rev.
Enable all the ttyUSB interfaces --> to get missing ttyUSB to execute AT commands and USSD
DWR-921 C3 modem comes out with a BroadMobi BM806U-E1 GSM/UMTS/LTE USB-mPCIe Module
(Qualcomm MDM 9225) but there is NO ADB interface. Maybe disabled by DLink
Do someone have a firmare from another device and or manufacturer (to try) to flash on such modem??
A Dlink's firmware even from a different product (with the same modem model) can be useful to conduct a search and dig about. (see: https://osmocom.org/projects/quectel-modems/wiki/D-Link_DWM-222_stick/10 )