Have you tried on another Raspberry Pi? It should have the same issue. Your ISP might be bound to the MAC address of the device. You can do the following:
- Call your ISP to release the MAC address and bind to the new host.
- Clone the MAC address of your Raspberry Pi.
What is the model of the modem? Is it Gigabit or Fast Ethernet? If Fast Ethernet, you also have to check if it has AutoMDIX. If it does not have AutoMDIX, you have to make sure you use a crossover cable.