After giving this project a rest for a month, I am on it again.
Today I successfully added the cypress_serial driver from teltonika to the FW, but, as I feared, this changed nothing.
I think the problem is somewhere within the USB stack. I only see one USB hub, but I think there should be two, if things were working correctly. At least that is what I see in the stock FW and on the RUT956 with the MeigLink modem.
Unfortunately I don't really know how to debug this further. I guess I'll have to find the right mailing list for an issue like that.
¯_(ツ)_/¯