Call your ISP and bitchcomplain about it.
This is a local-sided issue. Your ISP won't do anything but try and sell you a new router.
OP, create a static listing in the DHCP server for said IP with your MAC address/Hostname, or exempt said IP from the DHCP leasing.
You will have to port-forward no matter if your IP address is dynamic or static. The only way you don't have to is by getting rid of the router and connecting your computer directly to the coax cable.
No stuff? He's already port-forwarding, but is having issues with the DHCP server leasing his static IP.