Ok,
So this is what I would do, reserve that IP address on the modem so it doesn't change.
Access from the left navigation menu:
Connected Devices > Computers
To manually add a computer with a static IP address to your wireless network:
1. Under Online Computers, click Add Computer with Static IP. The Add Computer menu appears.
2. Complete the following fields in the Add Computer menu:
Option Description
Host Name Host name of the computer you want to add
Connection Read-only field that displays the network connection of Ethernet
MAC Address MAC address of the computer you want to add (use a colon between each 2
character ID in the MAC address)
Static Address Static IP address of the computer you want to add (use a period between
each octet in the IP address)
Comments Optional comments about the computer
3. Click SAVE to save your settings (or click CANCEL to discard them). If you click SAVE, the Computer
menu reappears with the computer you added displayed under Offline Computers.
4. To add more computers with static IP addresses, repeat steps 1 through 3.
5. To edit an online computer, click the EDIT button next to the computer you want to modify, edit the
settings on the Edit Computer menu and click SAVE.
6. To delete an online or offline computer, click X next to the computer. When the Delete Computer message
appears, click OK to delete the computer or CANCEL to retain it. If you click OK, the computer will be
removed from the Computers menu.
Then input that IP address into the DMZ on the modem. Save settings. I would reboot the modem and 4500 after the changes are saved.