I have 4 WiFi plugs that had been functioning well for several months. Now for no apparent reason they now regularly ( several times a day ) go offline rendering them and the cameras they control uncontrollable and useless v

I have done factory resets but to no avail. The only way I can temporarily guarantee getting back control is to reboot the router. 
I have been in contact with my internet provider and they can offer no solution. I have full fibre internet and the plugs are connected to the 2.4 ghz.

Any suggestions appreciated