Slow 5G periodically

Hi, So, I have a Nighthawk RAX35 router that suddenly is slowing down the 5G network, even in the same room. I have: 1. bought a new modem as Xfinity insisted I needed a new one, rebooted after the installation of that. 2. Set to factory settings and reset up the router. It did have high speed for less than a day after that. and 3. taken some stuff off of the router. I bought a Nighthawk mesh plug in to extend the network to my upstairs and it worked for a while (set at 5G to extender, 2G off of that). I live in an old house in NE with lathe plaster walls but my office is right above the router which is placed up high off of the floor and in the open. ONly a ceiling and floor between them, no wires, etc. Help, is it possibly a faulty router?