I also had that problem, I was trying to download and then seed something that was really only distributed on my LAN to other torrent clients and even trying to enable/disable local peering I couldn't get any client on my LAN or outside sources to seed from the location on my N66. All clients inside and outside could draw from the other clients on my LAN though, I could view their IP addresses.
This might be (one of the reasons) why running Transmission via Entware on the Asus is popular over DownloadMaster. If you search the threads you'll find instructions on Transmission. It's worth a shot.