Hi@Be_Fe,

Thank you for reporting your issue to our community.

To use the WDS feature on the supported routers, please follow the steps below:
    1. Log in to the web management interface of the router, go to Advanced > Network > LAN, and configure LAN IP address to be within the same subnet as the main router.
    2. Enable WDS and connect to the wireless network of main router. The WDS feature might be available on different pages for different router models, you could look for it on Advanced > System Tools > System Parameters page, Wireless > Wireless Settings page or Wireless > WDS page.
    3. Disable DHCP on Advanced > Network > DHCP Server page.
Please note that even though the WDS is functioning properly, it is normal for the Internet Status to display as disconnected. This is because the WAN port of the WDS is not being utilized. Meanwhile, not all TP-Link routers support the WDS feature. If you cannot find the WDS function on the web management page, then the router can only connect to the front modem/ router through an Ethernet cable. 
For more details, please refer to this FAQ: How do I use the WDS Bridging function to extend my wireless network?

  @Be_Fe 

If your main roouter IP address is 192.168.0.1, set C86 with IP address 192.168.0.254.

DH\CP on C86 should be off. Keep in mind that building WDS is not always possible.

Because this feature is not standardized, it depends on the main router make and model if it will be successful or not:

"WDS may be incompatible between different products (even occasionally from the same vendor) since the IEEE 802.11-1999 standard does not define how to construct any such implementations or how stations interact to arrange for exchanging frames of this format.-----From Wikipedia"