Hello,
I have been using WiMax Internet since 4 years, you actually need to change the location of your WiMax device to obtain new AIR IP from the tower it is connecting. If it connects from tower A a IP will be reserved for your device for like 24 hours (thats when the lease expires) now if you want IP other than this one then you just have to take your device to a location where tower B could give a new ip to your device.