Ok, I've had a nosy at this AP900 and looks like possibly a timing cap has dried out on the board so swapped them all out and seems to be working now, got a refund off the bloke I bought it from on ebay so will see if it holds up, fingers crossed.

Does this setup look right for the Routing policy?

https://postimg.cc/qzvKkSZK

You probably want the source to be "Any", rather than "IP Range".

You'll know if this bit is working, if you can access the AP900 @ 192.168.100.1, from the 2860's LAN.

(In normal circumstances, all data heading out of the 2860's WAN port, will be going to whatever is defined as the Default Gateway at the time. You just need it to use ARP to 'find' 192.168.100.1 instead. You'd think that the presence of that "WAN IP Alias" would be enough to tell the 2860 that there's a 192.168.100.0 network directly connected, but it isn't )

Should I manually configure the IP address of WAN2 to be 192.168.100.2 or something similar as this needs to be on the same subnet as the AP900?

I've tried the AP900 on 192.168.1.100, the 2860 on 192.168.1.101 using your example in the other post you included the link to but still can't reach the AP900's GUI from behind the 2860n LAN.

No. Leave WAN2 set to DHCP, so that it will ultimately get an IP address from the Hotel network (having set it back to wired-mode of course!). Set a "WAN IP Alias" for WAN2 to be something on the 192.168.100.0 network, for use in talking to the AP900. I gave 192.168.100.254 in my earlier example. In concert with the Route Policy entry, it will use that IP address when communicating directly with the AP900 (for management purposes only).

Is the reference to 192.168.1.100 in your last post just a typo?

I see what you mean now and yep, apologies, the 192.168.1.100 address was a typo.

What LAN address does the AP900 need to have (facing WAN2 of the 2860)? I was getting confused thinking the two should have fixed addresses, eg the AP900 using 192.168.100.1 and WAN2 interface 192.168.100.2 etc.

We're progressing... I've now managed to get the 192.168.100.1 management of the AP900 through the LAN side of the 2860n however I'm struggling to understand how to configure the AP900 so that it in effects allows the DHCP of the guest WLAN (hotel network) to assign this to the WAN2 port of the 2860n via it's connection to the AP900.

If I select DHCP IP assignment of WAN2 on the 2860n it doesn't assign an address from the WLAN the AP900 is sat on (using my home Guest Network SSID to test this).