"connection hijacked/frozen"

I have had the following problem twice this week.

1. My internet connection could be made only to a specific site(wenxuecity.com and its subsites),
and all other connections were denied with a message "Server not found".

2. Not even the Window Defender's virus definition could be up-dated then.

3. All devices, three PCs and one smartphone, were having the same trouble at the same time.

4. It lasted 3-5 hours and then disappeared. The same problem reappeared in a few days.

I have been using a 15-year old Motorola cable modem and a low-end TP-Link router. They seem to 
work just fine before.

What caused the problem? What might be a quick solution? I tried the power-recycle of the 
modem/router, but it doesn't work. Haven't tried to re-set the router yet.

Pls help. Thanks.