Thanks for taking the time to answer this question, which I myself consider myself old now. I found a viable workaround about 4 weeks ago.
Besides, that I’m surely not going to discuss this with the vendor of the modem. As we’re stuck with a highly patched ancient version of kernel 4.9 here, a “Maybe try a more recent kernel, pal!” seems unevitable.
To be a bit more constructive to any poor souls that might stumble upon this: I gave up and connected a RasPi with the modem, providing the network connection to the Nano.
Generally I consider the Nano being an appliance providing computing power for a specific AI job now, rather than acting as “the computer” on a project. Way to many issues ranging from problems powering USB cameras to faulty drivers for usbserial devices. If I’d have to guess: There are serious issues with the USB host controller on the Jetson Nano. But that’s just a rant. Good luck with your projects.