@Madgibbon 

Good day.

Thank you very much for your time and patience.

May I know the model number of your main wireless router?

Did you test both the 2.4ghz and 5ghz have the same issue?

Have you ever tested on different devices, like your mobile phone to see if they are always the Windows 10 computers which had the problem.

After that, please try the following suggestions:

1. Try to reserve the IP address for the computers on the main router;
2. Change the DNS server address on the main router to be 8.8.8.8 /8.8.4.4;
3. If possible, please also change the advanced settings on the router, like channel 11 and channel width 20mhz for 2.4ghz; channel 40 and channel width 40mhz for 5ghz;

Thank you very much and have a nice day.