Hello all,
I am wondering please if I am going one step too far with this.

I have a 2860Vn+ as a router and DHCP server.
Connected to this I have a PoE network switch.
Connected to the switch I have various wired clients and several AP900's dotted around to ensure good wi-fi coverage.
I have 2 different SSID's being broadcast by the AP900's and returning to the router via the ethernet wiring and all is well.
Each has its own IP pool, and all is well.

I have set the 2860Vn+ to have a 3rd SSID & associated IP pool as a guest network, and this works fine, but I don't have adequate coverage.

This is working and has been for some time, just patchy coverage for guests.

I want to add the guest wi-fi network to all the AP900's too, if possible.
My question is, can I add a 3rd SSID to the AP900's and get this to work down the wiring too?...
I can configure the intermediate switch as required, if necessary, I am first wondering if the 3rd SSID will work on the AP900, or is it a non-starter, please?

Any help gratefully accepted.

Long time since I had an AP900 but as I recall they can be configured with up to 4 SSID's - why do you have doubts that it'll work?

It has the ability to broadcast 4 SSID's yes.
When I enable the 3rd, I can't connect to it, all 3 are going via the PoE supply on Lan A.
I asked before I rip one out, and bring it into where the 2860 is and start playing around to see if it is the AP900, or the interposing network switch gear.

Well I have used 3 (sometimes 4) SSID's on AP902's, AP903's and AP960c's, all fed through a POE switch (Draytek then Zyzel) and all have worked, so I feel that yours should do so.....

Thanks, I’ll get a psu and connect directly to the 2860 then and see if that works.
I have a Netgear and a couple of D-Link units around, it could be the configuration of those units.
I asked because every document I had seen suggested two SSID's only.

Is this a VLAN-tagging question?, as per this thread: https://forum.draytek.co.uk/viewtopic.php?t=24431

My multiple AP's are connected via 3 Netgear switches to the 2860n. All are tagged-vlans and some confusion ensued, during their initial configuration

As an aside, I don't really understand the concept of "Guest Wifi", in conjunction with separate Access Points. For a simple, single 'consumer Wifi Router', "Guest Wifi " just means an SSID that gives a connection to the outside world, but not the LAN or other nodes. But the minute multiple APs are involved, with all traffic from whatever SSID having to share a wire back to the Router, doesn't VLAN tagging become an immediate requirement?