Why doesn't Casting work without internet?



So I bought a really big TV so my lady friend and I, and guests, could enjoy my video collection and DVR'd TV shows on something larger than my 23" monitor (designed for viewing at 3 feet). The TV is great, and it has casting built in, which is the only way I intended to watch anything on it. I don't have Cable or Satellite. I definitely don't want to run a 15+ft HDMI cable to my wall mounted TV from my PC.

But there was a storm recently, and although I maintained power throughout the storm, my Internet service provider turned off the Internet for my block for a couple days after it passed, to fix their network. And it was during this time without Internet that I learned PLEX can't Cast media to my TV when there isn't any internet connection!?

This is very strange, since the local network (LAN) was/is fine. And this was especially odd since Chrome itself can Cast mirrored Tabs or Desktops to my TV without an Internet connection.

This seems to be a limitation of the PLEX integrated Cast function.

Is there a good reason for it?

Is there a setting somewhere that fixes it?

I'd really really like to still be able to Cast local media to locally accessible networked hardware without an internet connection. Casting from Chrome itself still worked, like I said, but the quality of the image can be less than desirable while Casting (mirroring) Tabs or Desktops where there's a lot going on, like fast moving video.

Note: I run an up-to-date PLEX Server on Windows 10, and I have an up-to-date 2017 Vizio Smartcast TV.