Try to upgrade your FW to the latest one and see if that helps - https://static.tp-link.com/2020/202006/20200628/Archer_VR600v(EU)V2_200624.zip

@jcjc Hard reset (Method 1) the router and re-configure from scratch. Then check if that resolves the issue.

@timung Here's a referemce that you could use for configuring the WDS. Remember that IP of the C6 router should be in the same subnet as the main router.

Hard reset (Method 1) the router and re-configure from scratch. Test again and see if that helps. If that doesn't help and you don't wano to go back with the FW (if possible), then hopefully a new...

@HP15z Honestly, I don't know, since I have never used that feature, but you can browse through emulators of different TP-Link products and see if there is such an option. The other thing you can do...

Something like this device maybe.