WiFi extender

I have been having issues with my upstairs tv since I upgraded to a new Gateway. It goes in and out, pixelates, loses volume,etc. So I thought a WiFi extender would be the answer. I got a TP-Link Wi-Fi Extender. I set it up but my issue is with the bandwidth. I’ve never messed with that before. So they all have the same name and password and when I was finishing setting it up I chose 2.4 and that messed up my tv downstairs! I’m kind of at a loss. Any input would be appreciated. Thank you.