This is a basic TCP/IP issue - nothing specifically to do with the 8051 at all.
You would have exactly the same problem just using the modems with PCs!

There are various possibilities; eg:

Since you have SMS working, your units could exchange IP addresses via SMS;

You could subscribe to a service that gives you fixed IP addresses;

You could have a single station with a "well-known" IP address, and route everything through there;

Use DNS?

Avoid GPRS all together, and use data calls and/or SMS instead?

etc, etc,...