@Dail Have you tried updating the firmware on both? if yes tell me .... another option is to post here the log of both when the error happens ... could you?

@Andrsouzaaa 

Thanks for the reply.

Have been in touch with TP-link support who suggested

"Change the 2.4GHz's channel to either 1, 6 or 11 and 20MHz
Change the 5GHz's channel to either 36, 44 or 153 then set it to 20MHz too.

Once adjusted, power cycle the router and range extender and observe them together for a day.
Make sure that the SSID of the router and the SSID of the range extender is different from each other. "

But that didn't fix it and now assigned to a senior engineer.

Firmware is the latest on both. Have discovered that the modem/router had a log message for a warm reboot due to a crash, all the rest prior to that was the usual start up messages, see attached. Power cycle of the modem/router not required, just have to wait about 2 minutes for the wifi to come up and a further 2 minutes for the modem to resync. Then all back to normal. 

@Dail thanks ... @Solla-topee could you evaluate this log? I found some peculiar errors, there really is something wrong .... I hope you help us in solving this case .