I can confirm a similar problem with detection of Quectel EM160R LTE modem - MT3000 v.4.2 / GL GUI does not detect the modem, a new modem interface is not created, but the system log checked from Luci shows that the modem was found on the USB bus. The same modem works just fine on ATX1800 Slate v.4.2 after the fix provided (via remote access) by one of GL-Inet customer support rep :
https://forum.gl-inet.com/t/using-atx1800-with-quectel-em160r-gl/23443/37?u=i7bar
I was told that it would be added in v.4.2 but looks like it was not.