You’re using an unsupported tuner so your results may not be great. I’m gonna need the log from the client. Looks like you are using PMP in desktop mode, so get me the Plex Web logs. Plex Web App Logs | Plex Support
Oh sorry. When you said Gateway I thought that was a different model. Yes, the XTi-3452 is supported. That device is for a cable conenction. Have you checked if the channels you are trying to view are encrypted. Plex does not support encrypted channels. If you can provide me the logs I requested before so I can check the error, this may be the reason it is not working for you.
Unfortunately, without being able to see the exact error, it’s hard to say why things don’t work. If you don’t want to share publicly, you can PM the logs to me. Those are kept confidential and deleted once I’m done investigating.