Make sure you turn off the modem as well as the router. The only way you can change the MAC address is to go to Settings > Internet (Advanced Settings). Next to MAC Address Clone, select a device from the drop-down. This will have the router use the MAC address from the device you selected (computer, smart phone, etc) as the WAN/Internet MAC address. Save the settings. After the router reboots, unplug the power to your modem and router. If your modem has a battery backup, you need to remove the battery and let it sit for 5 minutes. Time may vary depending on your ISP.
Power your modem. Once booted, power on the router. In some cases, again depending on your ISP, you may receive a new IP address. If not, contact the ISP. They CAN give you a new IP address easily. Changing the MAC address in most cases will do the trick.