An unknown modem appears in network device list - evidence of tampering?
Run "ioreg -l | grep your search word"in the terminal maybe? Probably again without grep and just command F to search the modem name to see the things its associated with, if at all. That name with the two x's looks like things I've seen too. Sure there's more you can do but I'm no expert..just been dealing with a lot of odd things with Sierra myself. Are you doing this from single user or recovery?

If you're in recovery mode you should look for a bunch of ramdisks with "diskutil list" (15-18 about) if it's this embedded operating system firmware malware my devices all have.