firewall - access linksys router admin module remotely if linksys is behind a linux router

My linksys wireless router is behind a Linux router and I would like to access the linksys router's admin module remotely.

How do I configure iptables so I can open the admin module?

iptables -t nat -A PREROUTING -p tcp -i $EXT_IF --dport $PORT -j DNAT --to-destination $IP

Am i missing something?

Thanks in advance!