This issue is known my Glinet support since at least Oct.2020 (they spent time to capture trace on my setup), but since then, this issue is still not fixed. Basically, the behavior is similar to the issue with last IOS release, but the the latest firmware fixes only the IOS tethering issue, not the Huawei stick issue.
I duplicated the issue with all devices I have : MT300, AR750, AR750S, and with 2 LTE USB sticks : E3372h-320 and E8372h-320. Once plugged, the device is well detected, connected through Tethering, but no data can pass through (even ping 220.127.116.11), either from LAN or from LUCI diag tools. Admin interface of the USB-stick shows that LTE connexion is good, and diag from LTE webmin is successful.
The LTE sticks are working well on when hooked on computers.
Will Glinet fix this issue ?