It might be a bad connection between your computer and the router, but that’s a pretty low possibility. I think what I would do in your situation is , while the speaker is showing up in the controller, run a system diagnostic within 10 minutes of getting that error when you try to play something from TuneIn or Sonos Radio, and call Sonos Support to discuss it.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.