I know this info is somewhere but, I swear, I did search this forum and couldn’t find anything solid, only people struggling with various ones they bought. Can anyone recommend a few models (because I don’t live in the US and what is available here might be different) of 4g (sim slot) USB modems that work with the GL-AR750S-Ext with no fiddling? Bonus for modems that are not from Huawei, if possible. Thanks!
I’d suggest that you check https://www.ofmodemsandmen.com/modems.html
Find the “hostless” modems. Hostless modems works like tethering and should work with the stock firmware.
I just bought a GL-MT300N-V2-2e8 and I successfully connected to the USB port a Huawei E8372h-153 wingle (wifi-USB) that is recognized as Tethering and, when connected to a power adapter, as wireless repeater. I also checked a ZTE MF833U1 (USB Modem) recognized again as Tethering. I also have an old TP-Link MR3420 with ROOTER Firmware and both works perfectly connected to the USB.