To use a wifi router as just an AP.. You don't need to do anything with their nonsense interface... Just turn off its dhcp server - connect it to your network via one of its LAN Ports!!! Set an IP on this lan port to work on your network.
Most of these nonsense native firmwares do not even allow you to put a gateway on the lan side interface.. So no you wouldn't be able to get to it remotely from another network.
Put some 3rd party on it like ddwrt or openwrt... If that doesn't work an it will not allow you to put a gateway on the lan interface - then source nat it on pfsense so that traffic going to the AP looks like it comes from the pfsense interface IP in that network.