OpenWRT + 4G dongle/wi-fi modem


I'm using a GL-iNet MT300N-V2 mini router and a seemingly no-name USB 4G modem (its admin page is branded Qualcomm though).
The modem can work in 2 modes: as a Wi-fi router itself (of quite limited functionality), and just as a dongle - at least, this can be controlled via the modem's Windows driver.

The problem is, when I connect it to the router, no matter what mode I set on the modem before, it starts as a wi-fi router, and my MT300N has trouble configuring it.
I tried configuring a PPP connection to it via Luci interface, but with no luck.
Is it regular functionality of 4G dongles, and can I configure this modem with my router somehow?