Sometimes Router WiFi says no internet on some devices, not all devices . and at that time the C80 router's app login and web interface also cannot be accessed from From all those devices.

I have a tplink c6u router which set as WDS device to extend the range of my WiFi. When I connect to this c6u router all device internet working fine . But when connect to primary router C80 say no internet sometime. When I switch on off primary router C80 all device internet working fine for sometime. Please fix this problem ...... I have been facing this problem for the last three months.