No help to you but just FYI the latest firmware for the e.g. AP960 gives you the choice of using 2.4 or 5 for the mesh. I tried 2.4, which gave me a much stronger connection to the root (on 5Ghz I usually see 0% because there's a metal garage door in the way, but it still works OK). However, I did seem to suffer bandwidth issues on 2.4 so I reverted to 5.

Ok so have answered my own question which is a bummer

Picked up from Draytek knowledge base: The Vigor access points on the network must be connected to the router with a wired network connection to use VLAN tags required for a guest wireless network; wireless links such as WDS or Universal Repeater cannot pass VLAN tags that are required for a guest wireless network to operate.

Time to break out the CAT6 cable....

Thank for the input from you all.