SUCCESS! Hard to find out what's been the essential step. I just booted OxygenOs 10.5.7 this time with a SIM-card inserted and connected to WiFi immediately. Surprisingly now I could enable OEM-unlocking and after that "fastboot oem unlocked" without any issues. UT 20.04 is running, so the device will be my daily companion from now on. Thank you so much, mates! Didn't find out to mark that thread as solved.

@arubislander This thread I started was aimed to get insight in the possible porting of the OnePlus Nord. As such I hadn't planned to discuss the differences between Pinephone and Librem 5. And I shouldn't have posted my earlier comment, reading back I was wrong as I was referring to the binary blob for the Wi-Fi/Bluetooth issue still in the back of my mind. I'm sorry about that, the last thing I want is to keep any misconceptions alive. The last sentence has been striked through and apologies to @PINE64 are in place.