As @StreamerUser mentioned, each Roku TV manufacturer provides direct support for their products running the Roku OS. Have you tried contacting Onn support yet to inquire further? I'd suggest starting there to see what options or suggestions they can provide.
Have you tried just taking the TV back to the Walmart where you purchased it? I have always had great luck with Walmart as they have one of the most liberal return policies on the planet, with and without a receipt. I would sure give it a try before doing anything else. They will most likely just give you a new TV.