Yes, that is the way GPRS networks usually work - in fact, it’s the way most internet connections work.
If you want a fixed IP address, you will to pay for it.
It’s nothing to do with Wavecom.
The IP address is assigned by the GPRS network.
To initiate a connection from your PC to any IP node always requires that the node has an IP address visible to the computer;
Conversely, to initiate a connection from any IP to your PC node always requires that the PC has an IP address visible to that node.
This is standard networking stuff - nothing specifically to do with the Fastrack, Wavecom, or GPRS.
See: viewtopic.php?f=5&t=1809&p=6793&hilit=IP+address+tutorial#p6793
And: viewtopic.php?f=39&t=1834&p=6775&hilit=IP+address+tutorial#p6775
Because your PC and the Fastrack will be on different networks, you will need to get a Public IP address for your SIM - if you want the PC to initiate the connection;
Conversely, for the Fastrack to initiate the connection, you would need a Public IP address for the PC.
Alternatively, you might want to look at so-called “dynamic DNS”; eg,