I had the same issue until recently. I was using two firesticks that were approximately 8 years old on each of my ROKU TV's. I replaced the firesticks with two brand-new ones and instead of powering the firesticks with the TV's USB port like I did previously, this time I used the included AC power plug and have not had a wifi disconnect ever since. My guess is that it is not the TV that is causing the problem, but most likely the firestick and or its source of power, if you are using one.