@Antibiotic So your using some wifi router as just an AP.. Yeah if your just using the makers firmware many of them do not allow setting a gateway on the lan interface.
Can you put 3rd party firmware on it - ddwrt, openwrt, etc. All of these allow for setting a gateway on the lan interface.
If not - sure you could either nat and and allow remote access to its web gui. Or as mentioned you can do a source nat, ie outbound nat so traffic to the AP IP looks like it came from pfsense IP on the same network.
If me I would look to 3rd party firmware. Or better yet get an actual AP vs using some wifi router you had laying about.
Advantages to 3rd party or actual AP is you would also then get vlan support, not just gateway on lan interface.
If stuck with this wifi router as your AP, I would use the outbound/source nat vs double nat and remote access to the devices webgui from remote.