If you have a 2.4ghz only Roku device, and Xfinity as your ISP, try changing your "Wireless Mode" under the 2.4ghz Wireless settings in your router to b/g/n.
However, if your Roku device is having problems staying connected to both your network and your mobile hotspot, it may just be on its way out.
Try powering from a wall outlet with USB power adapter (any 5volt, 1amp cell phone charging adapter should work).
Feel free to post back with update and model number of your Roku device (Settings/System/About) if still having issues.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941