Thanks for the updates, @JaredG24
We see that you're able to connect to a hotspot but still having trouble connecting to your router. In this matter, we recommend to continue working with T-Mobile's support since this issue is rooted in your network connection as the Roku device was able to connect to the hotspot. Mostly, it has something to do with the network's settings and configuration to allow it to be detected by your Roku device and the internet service provider's support can assist you further.
We hope this will be sorted out soon.
Best wishes.
Kash
Creating another network on the TMobile 5g was a good step to try and was going to be a suggestion after confirming you could connect to a hotspot. (confirms that the Premiere radio/module is working inside the Premiere).
I don't have any Arcadyn KVD21 routers to try but i have a newer Arcadyan G4AR model (for TMobile) and it is recognizing my Premiere 3921 without issue.
Since you had issues with multiple Roku devices and this router, I think you should just ask TMobile to send you a new 5g Gateway to see if that resolves the issue for you.
Feel free to reply back if you want to continue troubleshooting but I think we have to eliminate the KVD21 as a culprit and its 2.4ghz radio capability. It may just not be broadcasting even though its diagnostics screen says it is enabled. Maybe they can send you their newer model.
It's interesting that you said "radio" because there's an error message that IT said was a radio signal. What if that signal is coming from the Roku?