To elaborate on what saltyNZ said, 2deg on the default "internet" APN no longer give out public IP addresses, instead you get a private IP in the 100.x.x.x range and they do NAT to translate that to a real IP address.
This will very likely be the cause of your VPN breakage.
You could use the "direct" APN which would give you a real IP address, but it will also not be firewalled so you could potentially be billed for data you didn't initiate.
I'd really like to see 2d offer a compromise here. Maybe paymonthly users get a real IP and prepay get NAT? Or another APN we can use. NAT is a pain.