I have a HDHomeRun that my Shield has no issues seeing in the Live Channels app via the HDHomeRun source but I can't get the network TV tuner to show up.
What I've seen before is make sure there is no attached storage. Also try uninstalling updates, clear cache/data, install updates, open close open.
EDIT: at this point I'm guessing my model is not supported.
EDIT2: Turns out my model isn't supported. Its the gen just before they started adding DLNA.
Goto “Customize channel list” and see if the Network Tuner is listed and Select Group to Activate Network Tuner.
Under Settings, Channel Sources, if you don’t see “Network TV Tuner (BETA)” it should be listed if Live Channels is working properly.
Reinstall Live Channels to repair any anomalies encountered.
To test Live Channel “Channel Sources” please download PlutoTV and select and import as a source.
The Live Channels guide should now include all of Pluto’s TV streams.