@asnikifor 

WDS may not always work due to the fact that different manufacturers may use different WDS standards, and they may not be compatible with each other, terziyski explained this in the following thread:

https://community.tp-link.com/en/home/forum/topic/568574?replyId=1111102

When using another router to connect to the Archer C9 wirelessly via WDS, there should be nothing needs to be modified. Just make sure there are no access control settings on it that might block the other device to connect to it.

You can try to call the mercusys support to be advised further.