I recently bought the onn streaming player just to see how it worked, and I’m mostly rather pleased with it. Except for Plex.
I got Plex installed and signed in and through the streaming services step. It’s now sitting with the spinner going at the Home Screen, with nothing coming up. I’ve been letting it run. It’s been 20+ minutes so far.
I use one of these (the 4K version) on a 1080p TV. It’s actually become my go-to Plex client recently. I don’t recall having any difficulties in setting it up, but you can try the following to see if it kicks yours into gear.
In the Onn device’s app settings, select Plex and force stop it. Then, clear the Plex app’s cache as well. Don’t clear data unless you want to have to sign Plex into your account again (which you may eventually want to try).
After doing the above, open Plex again. It should either land you on your home screen, or ask you to pick your default server and pin your libraries. If it doesn’t, it may be that it’s having trouble connecting to your local Plex server. If that’s the case, I’d ensure they’re connected to the same logical network (same subnet or VLAN) if you’ve divide yours into multiple.
I restarted the device and tried again. Plex didn’t give me the server select and isn’t seeing the main server, but that might be the server having gone toes up as it often does (the secondary server also does this.)