Model : Archer C9

Hardware Version :

Firmware Version :

ISP :

I think I know whats happening, but I'm looking for guidance or advice or even expertise. This is my setup;

Archer C9 connected to the internet in my basement - I have three wired connections with it, another router on the top floor of my house that serves as WiFi up there, I also have my Hue bridge connected to it and an iMac. Other than that, it's all WiFi downstairs, Chromecast, two Sonos speakers and roughly 5 or 6 devices at any one time.

This is what happens:

I sometimes lose WiFi on Archer C9 (the one connected directly to the internet) but not internet access. WiFi on other router is fine, Hue bridge is fine and iMac all works fine - connected. But I see the signal for my downstairs WiFi from the C9 but there is no internet access - only on WiFi. If I restart, its fine.

Doesn't happen every day, but 3 or 4 times a week and it's very frustrating. Only starting to happen last month or so after having for a couple of years.

Any suggestions?