Relatively simple to do. On the second (5GHz) router, change its IP to a static IP in the same network as your primary router (and not otherwise part of its DHCP pool). So, for example, if the primary router is 192.168.1.1, perhaps make the second router 192.168.1.2. Now disable the DHCP server on the second router and patch it to the primary router, LAN to LAN. That's it. You now have a second WAP that uses the primary router's network, DHCP server, firewall, etc.