My router (PC-Engines ALIX) runs on 192.168.111.1 and does all the normal router things plus VPN and USB. It has no wireless.
I have a TP-Link WDR3600 which I would like to be an AP with 2.4G and 5G "internal" (all access) WLANs (done i think). I also would like to create an additional Guest WLAN on the 2.4G band that only offers internet access.
As I understand the Dumb AP with Guest recipe, it's basically creating a DHCP server (in my case 192.168.31.1) for the guest, isolates it from the primary LAN and sends the traffic through the TP-Link LAN to my primary router and then the internet.
I think I am basically trying to do what the OP of the original post wanted to do, but his primary router was his ISPs if understand his drawing.
I did freshen up the ports as you suggested with no joy. Devices connect but get an IP in the dreaded 169.254 range.