This is my first server that i want to share out of my home network.
It is much easier to share the disk that is connected to the broadband. You should change over your present arrangement so the server you want to share to WAN is the primary router. The old existing TC which I guess has been taking that job should be bridged.
You can then get WAN access.. but you might need to use Lion plus iCloud. Unless you have a public IP.
Please first of all download and install 5.6 utility as i guess you are running Lion.
http://support.apple.com/kb/DL1482
Then you need to figure out how you are going to access WAN port as there is one key issue.. your public IP from the ISP is more than likely dynamic and keeps changing.. so you need to use BTMM from iCloud connection ..
Or you need to buy a static IP from your ISP..
Or you need to use a different router that is vpn server capable. Ie you must put both TC in bridge and replace it with a proper vpn router.
To help with specifics you need to tell us what kind of broadband you have, what modem you have.. and if your ISP offers static IP.
Do a google search Time Capsule remote access.. there are plenty of posts with explanation of how to do it.
The official Apple way.
http://www.apple.com/au/support/icloud/back-to-my-mac/