Is the TV on a wireless connection? If you are able to bypass any router and connect the TV directly to the modem for testing that would help with the troubleshooting.
Buffering on the app is nearly always related to connectivity issues.
How is this post marked as "solved" when it isn't. I would say it's the Insignia TV that's the problem. If it streams fine from the X-box, it's the TV.
I would agree that the TV is the problem. Or, if my recent experince is anything to go by, the quality of the Wi-Fi signal reaching the TV. I would try to improve that first - my own Roku enabled TV was having streaming issues with the Spectrum app (all other streaming apps were just fine) when the Roku in it showed signal strentgh as only "Fair". Things have improved dramatically since I was able to get the signal strength up to "Good", which admittedly took a lot of work AND the purchase of a more capable router.
17 out of what? What plan do you subscribe to?
If you are subscribed to anything over 20 I would have to say that is not a good result.