Fixing Actiontec Verizon FiOS router and Chromecast

There has been a Chromecast running on my family's FiOS connection with the Verizon-supplied Actiontec router for months now. A recent reboot of the router made it impossible to get the Chromecast successfully connected. The end result: We couldn't see the Chromecast from any devices, and a Chromecast factory reset left the Chromecast unable to successfully re-connect to the network for setup.

The router involved:
MI424WR-GEN2 Rev E
Firmware 20.21.2

After multiple retries and some research I ended up on a Verizon forum where I found I'm not the only person with this issue. There were a slew of suggestions to reconfigure the network, but the one method I found to work for my setup was this:

My theory is that the reboot triggered a settings change of some sort that was pushed down by Verizon. Frustratingly, this was not an issue with the Chromecast until the reboot of the router. The setup issue persisted across a Chromebook and 3 different Android devices on the network. For whatever reason the IGMP page doesn't seem to be accessible unless you know the page number to go to. Thankfully my parents Chromecast is now up and running again; hopefully this helps out someone else encountering the same problem.