Using your new antenna on as short and thick of a coaxial cable as
possible, set up the first DD-WRT router as a client to communicate
with the distant link. Learn about how to set it up by going to DD-WRT
site and folowing their tutorial for using DD-WRT in client mode.
Once you know that it will work, plug the router in client mode into
your 2nd router, which does not need DD-WRT and use it is as a regular
wireless router, with it's own firewall. Be sure to use security on
the second router.