@Riley_S 

Thank you for your answer.

Are you in the US, using a US model router?

 

I am in Canada.

It is possible that the prompt is being thrown because you are trying to use DFS channels for the 5GHz, which will bring your 5GHz network down while there are official communications happening on that channel. You may try changing your 5GHz to use channels outside of the declared DFS channels. It may just be that the enabling of WDS causes this check to occur more frequently or changed the channels to communicate between devices. 

The main router channel is set to auto. It always chooses 36 or 161, so it doesn't seem like this is the problem.

Lastly, have you waited a longer period of time to see if the 5GHz connections will eventually rebuild themselves or do the other APs stay disconnected until the rotuer is rebooted?

 

I have waited a couple of hours with no success. After failing, the "5Ghz Status" shows

Wireless Radio: On
Mode: 802.11a/n/ac mixed

Channel Width: Auto

Channel: Auto (Current channel 1) -- normally 161

WDS Status: Assoc -- normally Run

On the main router, the channel is still 161 (not 1 !) and the Archer C2600 doesn't show up as a connected device. I have no idea what triggers the disconnection, but It looks like a bug to me.

I discovered that the bug seems to only happen when both routers (main and bridge) have the same SSID. I have not seen any disconnection so far (24h+) if I choose a different SSID on the C2600. This is not as practical as having a single SSID but at least I don't have to constantly reboot.