I've been bopping around in the ZyXEL hardware forum looking for answers to this problem.
I think that before I try the new modem angle, I may see what happens when I set the modem to a static IP address (though I'm not sure how to do this yet).
My logic is, it appears that the modem is going out and requesting to the DHCP server every 15 minutes or so. The DHCP server is what dictates the lease duration time, but I don't know if there's any way to know if it's giving it a 15 minute lease, or if my modem simply requests a new one every 15 minutes.
I'm wondering if setting the modem to a static IP versus dynamic would shut down this routine?...