I did the 'Turn off DHCP an LAN port" trick and I can manage the router , no problem. The router is fully accessible on its own IP. Yes, some features do not serve any use as an AP, so them not working is fine. You use it as an AP because you already have a modem/router that takes care of firewalls ect.
So what's opposed to this solution?
And I do not understand this: "Perhaps setting the wan side to a different subnet and allowing the lan side to be on the same subnet as my new router".