Router 2gHz WiFi signal is erratic, the 5gHz signal is fine

We have been having trouble with the 2gHz WiFi being erratic. I have a WiFi monitoring program and it reveals the router 2gHz signal is varying all over the place. It can be fine and then it will drop 40 or 50dB. The 5gHz WiFi signal is strong and rock solid. I have a desktop PC that cannot get on 5gHz. I cannot get through to Comcast to get a replacement router. The "assistant" cannot understand what I am telling it. Help!