outlook, vpn, wifi and hotsport

I have a user that travels about the UK with a laptop

they connect to our server running exchange 2003, all works ok in the office

They use a 3G dongle and this connects ok using VPN to the exchange server with outlook 2003

if they use a wifi hotspot it doesnt work or if they use it on a different wifi router

at their home their router was setup with the same IP scope at the server uses 192.168.16.x

why cant i connect with a router with an IP addrss off 192.168.0.1 etc