Issue Updating Firmware with OpenWrt [Linksys E8450]

When I attempt to update the firmware on a Linksys E8450 with OpenWrt, it gets stuck.

When I click on "Start Upgrading", it just gets stuck in the 'Please wait' state. I am attempting to create a subnet within my home network. I have an Xfinity Gateway that acts as a modem and a router, but I want to create a subnet that firewalls the Xfinity Gateway and allows me to run a private server.

I have already changed the IP address of the router to I am upgrading the router using a Mac and an Ethernet connection. My internet is still actively connected to the Wifi provided by the Xfinity modem / router.