There are so many possible problems with WiFi, hard to know where to begin. I would create an SSID that is only on the 2.4ghz band and another SSID that is only on the 5ghz band as starters. If the router is picking channels automatically, rebooting may result in a different channel. Do you have any type of WiFi scanner that will help you pick relatively free channels? On Windows, WifiAnalyzer from nirsoft.net is great. On Android, there are many similar apps.
Just curious, what firmware were you using when the Wifi was good?