I've just purchased a AP905 to use as a mesh node with the 2866ax as the mesh root.  I've tried setting up the mesh network using both the router firmware as well as the Wireless App but the AP905 only shows as yellow Online light and not the green Online Sync Ready light. So it means the AP905 hasn't received the SSID settings etc.  The intent was use the Mesh wirelessly rather than through POE.  The user guide says to setup the node using a wired connection until the settings have been synchronised but I've left them connected (by wire) overnight and it hasn't synchronised.

The 2866 in using 4.5.2.2_BT firmware and the AP905 on 5.1.0.1 firmware.  I did try to downgrade to earlier firmware (using the compatibility table shown in the router control panel) in case there is an issue with the latest firmware but I had the same results.  One thing I've found is that going into the Mesh Status page on the Router Admin control panel and clicking the Password button that when you try to enter the AP905 credentials nothing appears to happen (there is actually a javascript error and I've cleared the browser cache to make sure it isn't using an old page).

I enabled the syslog on the router and noticed this message:

[dmn] Postpone Auto Reselect since Config Sync is not ready.

I have a related? issue with an AP905 node and a 2927ax root (wired connection). Although the "Mesh Status" shows as green (sync ready) for the 905, the CFG Sync remains at "Ongoing" and never syncs. Manual setup is required. My three AP906s all sync fine.
Seems that these products are not (yet?) as compatible as Draytek say they are.

I found three different access point compatibility lists for my 2866 and they all said different things.  It would be helpful if DrayTek could purge or update out-of-date information, or at least put a creation date on the web page.  So other people don't get led astray as I did I'll put a link to what appears to be the best list here:

https://www.draytek.co.uk/information/our-technology/mesh-wireless-compatibility-list

The other two, inferior, sources were the access point support list in my fully up-to-date (FW 4.5.2.2) 2866Vac's management interface which fails to mention the AP918R and a table on a web page which lacks many recent products.

It's interesting that upgrading the 2866ax router firmware from 4.4.2 to 4.4.5 appears to cause it to lose support for the AP906 and AP1062C access points as mesh nodes.