I'm taking it you have a router with a USB port that allows a drive to be shared over a network.
Unless you have Truecrypt running on the router itself, the router isn't going to know what to do with it. It will treat it like an unformatted disk or a disk with an unrecognized filesystem.
Furthermore, Truecrypt works on the block level. I'm betting your router basically has
samba or something similar installed that implements Windows file sharing - hence your router is sharing on the file level. So your other systems are connecting to the router's exposed samba/CIFS share. This is vastly different than how Truecrypt accesses the drive. Truecrypt cannot mount a volume via samba/CIFS. (It does work over iSCSI - I won't get into it now but it won't work how you likely want it to work.)
If the router was running Truecrypt, then it could share the device by mounting the Truecrypt volume itself and then sharing that mounted volume over samba/CIFS.
You may want to setup a separate PC with Truecrypt installed and use that to share the device over the network.