VPN

Good Day, i already installed a VPN connection for our three sites, and the main server at the main office, i can access all the computers at the other sites, but my VPN clients can only access the server, which running ISA 2004, and am using TrendNet VPN routers, i can ping the WAN Ethernet IP but cant go after it. i tried to add route from my VPN clients to my WAN Ethernet IP which is Routed to the LAN Ethernet IP. but still cant access it or access the LAN clients. any help! or idea to solve this out.
thanks in Advance.