Unable to add access points to mesh network

Hi all

I recently blew away my UTM and replaced it with XG.

Generally I have to say I am very imrpessed. I do miss the integrated AV but that's not the point of this question.

One of the reasons I made the switch was the neverending game I had of AP's going inactive on the UTM. To date, that hasn't happened on the XG (touch wood!)

Now though, I want to rebuild my mesh network.

Currently I have three AP30's configured on the same SSID, bridged to LAN, all on the same channel and all on the 2.4GHz (only one available on a AP30).

I've tried enabling STP as I thought I'd read that was a pre-requisite somewhere but this happens regardless of whether it's enabled or not.

I create the mesh network with it's unique name but when I try to add access points, the drop down box that appears for search / add is empty.

Just out of curiosity, as I know the docs suggest using a different frequency for the mesh to the SSID I tried creating one on 5GHz and get the exact same result.

All the AP's in question are active and indeed, I am connected to the WiFi to type this.

I'm at a loss to why it's doing what it's doing - the logs don't give any info.