IT Guy here. You have a wifi problem. I have a very nice WiFi router myself... Asus RT-AX86U and even it will make stupid channel choices if left in auto mode. If I force the 2.4GHz and 5Ghz radios onto the right channels, it has amazing range and speed.

You probably need a better and well configured Wifi router. You also need to do some speed tests throughout the day/week to ensure that your internet provider is actually serving you the 500 mbps that they are charging you for. For people that don't like to fiddle with routers, I usually recommend replacing the whole system with a two or three node mesh network. TP-Link makes a pretty decent one that won't break the bank.

If you just want to solve this FireStick/YTTV buffering problem then the cheapest thing to do is run an ethernet cable to your firestick. Honestly, spending a little money to have your expensive internet connection cover your entire house properly is worth the time and effort but I understand most people don't get as focused and picky about that as I do.