How far away are you from the VPN host server? This can have a drastic affect on your ping times and whether or not your connection is stable.
As for changing the IP address, not possible
. Your 3G device will receive a public IP address from the 3G provider and therefore you cannot assign a different one. If it disconnects in different countries then I would be almost certain that this isn't an IP issue since each country will give a different public IP address.
The reason I suspect an unstable connection is because you say it connects for 5-10 seconds then drops. This is a classic symptom of VPN protecting itself from a packet sniffer which is determined by packet times and packet loss. That being said, it is possible for the VPN server to reject you. Do your currently used credentials work via a wired connection?
I think the best person to go to is your tech support team since they are the ones with the most amount of domain knowledge. Tell them that you have tried in multiple areas and they should understand.