Does the NOK 5G21 Gateway have a bug in the LAN connections?

The OP asked about the Nokia, so my reply was about that. Sorry to hear that the Arcadyan has that problem. I was going to see about getting one. If any other Arcadyan owners are experiencing this problem, it would be good to get a thread going about that gateway.  Perhaps you could post your experience, Unix_User, under a new thread with Arcadyan in the title. I also have an ATV, and it streams 4K videos perfectly via the ethernet (into a switch, then to the gateway via ethernet cable).

The problem you describe can also be caused if your ATV had a static IP address that you were using with your previous router.  The T-Mo gateway uses a 192.168.12 subnet that none of my previous gateways used (cable or DSL). So, for example, if your ATV had a statice address in a 192.168.1  or a 10.0.0 subnet, then the gateway would never see the ATV.  I've read, but not experienced, that some ATVs had trouble getting a dynamic IP address and needed to have a static address set for a subnet used by the router. I only mention these latter two causes to suggest that it may be something with a setting in your ATV, rather than a bug in the gateway. Just something to check.