Telnetting into Mainframe server via VPN (HELP)

[Andeeee]

We have a Cisco Concentrator 3005 set up for our vpn connection, when we connect using the cisco client we are connected and authenticated from the radius server.The problem we have is that we can't telnet into the server or ftp but we can ping it from the clients, there is a firewall inbetween and the relevant ports have been opened i.e. ports 23 and 20/21.We have (to narrow the problem down) put a box between the concentrator and the firewall and can telnet to that as well as telnetting to the concentrator itself, also telnetting without the vpn client is possible to the destination server.
The firewall is a cyberguard.We understand that as it goes through the vpn the packets are fragmented and could possibly not being seen as valid packets when they get to the firewall.We are aware of the mtu utility that comes with the client and have tried increasing and decreasing the packet size to no success.
Would really appreciate any help or advice

Thanks Andy