Router Blocks Access To Single Site, Only On Selective Devices

I hope this helps someone else who runs into this issue.

My problem was finally solved with an option in the Tomato firmware.

There is likely a way to do this without installing the Tomato software, but I do not have knowledge on where the option is in the stock ASUS firmware.

The Fix:

The option that fixed it all was setting the Route Modem IP to the address of the Modem/Router that comes with the ISP. In my setup, I connect this ISP modem/router to the WAN port of my ASUS router and disable everything on that ISP router (like DHCP etc...).

For my specific case, the modem's IP (the IP address I use to get to it's web interface) was 192.168.0.1. So I set that in the text box.

I was literally just trying every option and meticulously resetting everything in between each attempt. But as soon as I changed that option, every device was suddenly able to access the site.

I apologize for not being able to provide an explanation of why this worked, although I can confirm that setting that option back to Disabled suddenly breaks everything again.