You can try this if you like.
1. Create an L2 VLAN on your controller, say vlan 888. No VLAN interface needed
2. Create a wired AP profile and assign it to vlan 888, bridge mode, access
3. Create a AAA profile and make the inital role to be "authenticated"
4. Create a wired port profile and assign your new wired AP profile and AAA profile to it
5. Assign one of the ports in the RAP-5's AP group to your new wired port profile, connect the LAN port of your DSL modem to that port
6. Create a guest SSID and a AAA profile with initial role "authenticated" (this assumes a PSK SSID)
7. Create a VAP, bridge mode, VLAN 888, assign your new guest SSID and wireless AAA profile
8. Assign the VAP to your RAP's AP group
9. Make sure your DSL modem is acting as a DHCP server
Save the config, give it a whirl. You can try to tweak the initial role if you like to lock things down a bit, but I just used "authenticated" in my lab for test purposes.