Background info
DHCP gives you an IP address with a lease time.
If the device with the same MAC address requests an IP address within this lease time then you will get the same IP address again.
What i did is scheduled a job in Windows with the AT command to ping
www.google.com at regulair intervals within the leasetime periods. This will keep the line active. I am doing this for more then six years now and still got my IP address as if it was a fixed address.
hope this helps
Bram