My recommendation would be the following:
Keep the EAP120 disconnected from your upstream router for now -- just an ethernet connection to your computer for admin.
- Start by performing a reset on the router to get it back to defaults. It will reset to 192.168.1.1
- Log into the router and enable wireless with your desired SSID and password.
- make sure you can log into the wifi, get an IP address via DHCP, and connect to the router admin page and/or ssh. (do this with another device and/or disconnect the ethernet cable between your computer and the EAP120.
- Change the IP address of the EAP 120 to the desired IP address (IIRC, 192.168.5.5). If you do this by LuCI, keep in mind that it will auto-revert unless you can somewhat quickly get a new DHCP lease and connect your browser to 192.168.5.5 to accept and commit the change.
- As an extension of #4, make sure that you are getting a valid IP address in the 192.168.5.0/24 network and that the router responds at 192.168.5.5.
- Turn off the DHCP server.
- Make sure that you are no longer issued a DHCP address on your computer (renew the lease, it should fail).
- Set your computer manually to an address in the 192.168.5.0/24 network and ensure you can still connect to the EAP120 (test via both wired and wireless).
- Connect the ethernet to your main router (use the same ethernet port you used for all the admin stuff), and now see if you can get a DHCP IP address. It should be in the same range. Make sure that the DHCP supplied DNS and gateway are correct (192.168.5.1).
- Test to see that you can connect to the internet from your computer.
EDIT: basically the above is taking the dumb-AP approach step by step, but testing along the way.