I have a broadband that here you can see its details. It provides wireless and also I got two more routers that use them as access points. DHCP is disabled on them and these two routers work are set by static IPs. Since the beginning, every few hours all connected devices disconnects for 1-2 minutes and reconnect again. It is like the main router restarts or something like that. It does not matter what device is connected to what router. Everything disconnects all together.
Clearly the main router is the problem but do you why that happens?