You will need to contact AT&T as I don't think this is an option for 'test' accounts. I believe all of the operator trial kits on the M2M.com site by default provide dynamic IP addressing because ordering is automated. If you need a static IP address, the operator needs to create a new account for you and move your SIM's to that account. This new account will use a different communication plan so your SIM's are allocated a fixed IP address from an agreed range.
Whether it is via a public or private link, and any changed to costs is the other thing you need to agree with them.
Depending on your use case, an alternative may be to use AirVantage M2M Cloud to communicate with your GX remotely. You won't need a static IP address in this case.
I recommend you to register a trial account for AirVantage (free of charge, limited to 5 devices though) and register your device to your trial account with the "Register" menu in AirVantage, once you'll have received your account.
You'll find more technical information regarding Sierra Wireless products on developer.sierrawireless.com
Would you need more support, feel free to ask on Sierra Wireless Forum.