The default gateway must be the host on the same network which knows the route for other networks (because he is directly connected to another network), so for your dd-wrt which is connected to the main AP it would be the default gateway of the AP network for that interface, as for your hosts on the dd-wrt that should be the dd-wrt itself, just think of the dd-wrt with two interfaces, one that faces the main AP and the other one who faces your internal DD wrt hosts
Your main AP should be already configured with the correct gateway, otherwise it wouldn't even get to the internet, and the dd-wrt itself with the interface connected to the AP should be good because it probably just got a DHCP address with the correct information
I said DHCP pool because I assume you configured the addresses on the host you connect to the dd-wrt, which contains all that information such as the IP address you want to assign, DNS servers, default gateway, etc