Thank you everyone for the advice! So far I cannot fully test the USB part: my Lenovo laptop doesn’t let me choose between USB 3 and Legacy modes, and I have only one micro-USB cable to test the device with.
I did a passthrough of Jolla C to several VirtualBox machines configured to use “USB 2 Control (OHCI+EHCI)” and running Linux. I think that did a similar thing to what @Seven.of.nine suggested: use older USB2 over real USB3. No luck so far.
I guess I need another PC, some other micro-USB cables, maybe try Windows or macOS. Still not sure if the issue is about connections or something else. I’ll be back when I have anything more to report.