Earlier quoted context omitted.
An ethernet usb dongle that an ISP tech support guy is likely to have is more likely going to be a single purpose translator without upgradable firmware (because this makes it the cheapest possible, and these types of devices rapidly fall to the "cheapest possible" price point). You also did not say what OS you are running on your laptop. If it is any later version of MS Windows, then you have infinitely more to worr…
He had an Anker dongle IIRC (and I run Debian or Arch, depending). I think the BadBIOS episode infected me with that security researcher's (apparent) paranoia.
Then you are most likely (as in 99.99% likely) simply being paranoid for nothing.