@terziyski I used the block list feature within the router. Unfortunately I've already tried looking up the Mac address but it resulted in unknown device. Tried multiple sites but no go.

The device keeps downloading more and more data according to the Tether app, but using the web interface there are no unknown devices on the network. As such my current conclusion is that there must be a bug with the Tether app.

  @Nertip It definitely is a firmware bug. I'm using web UI and also seeing this. At first I thought someone got me. Then I realized my WIFI password is a pain to type, and would take more than a thousand years to bruteforce according to Kaspersky's and BitWarden's password checker. So I went back to really pay attention and noticed the mysterious device comes up every time the router got a hold of a WAN connection. The uptime is about 1 second away after WAN is 'UP'. 

Mine shows as connected to 5G band. Blocking this device or not doesn't affect the internet usage at all. If you keep it blocked, it won't show in the web UI clients list, but would come back after the router is rebooted.