Amazing. I have the exact same problem. Switched ISP and got a new router/modem from them. The new router doesn't pick up the DNS-323 when I plug it in so I thought I'd keep my "old" LAN on the DIR 635. And I can't bridge the new router (Thomson, Telia for you Swedes) since I use TV and PHONE over the "internet".
It really worked well until I tried to get FTP working, so I have the same problem. Just thought it was to direct port 21 to the old router and then do the same from that one to the DNS, but no. Will try to eliminate the NAT layer, don't really know how I do that but I trust Google is my friend, then and let you know.
BTW, putting the second router in DMZ, is that such a good idea, security wise?
Feels like i'm way out of what should be posted in this forum, as for the subject, but we're one big happy family, right?