Hello
To implement offline hotspot, you need to install dns server on your dd-wrt system.
Normally DD-WRT has dnsmasq as local dns server and forwarding, and also dnsmasq works as dhcp server.
On the dnsmasq configuration, write address filtering script that redirect all dns addresses to your dns and on the iptables firewall redirect port 53 to your router's.
That is all, it is easy to implement.
Thanks, hope it helps for your project.