Hello
We can help you but we need more information about what you're trying to accomplish. What does your network look like? What is the topology (current and desired)?
From what you wrote I understand that you connected to your ISP by using the NetGear DGN 2200 router. Do you want to replace it with a Cisco 881, or do you want to terminate the ISP link on the 881 and then connect the NetGear to the 881 as a LAN router?
Best regards,
Martin