I realize this is probably a common issue but the threads I saw on a google search were mostly from 2018. I have a TCL 43" and my gf has a TCL 55", both of which I believe are 2019 models, and both are running fully up-to-date.
For my house: Using pixel with youtube, regardless of if I'm on my 5gh or 2.4gh bands I can see and connect to the TV. Using the Screen Mirroring from Pixel, again regardless of 5hg or 2.4gh on my pixel or TV, the android device cannot see the TV. Similarly my wired desktop on Windows 10, or my laptop on windows 10, both cannot see the TV regardless of which network configurations I choose.
At my girlfriends we have it set up for a single SSID. Windows 10 can see the TV but times out trying to connect, no prompt appears on the TV. On pixel, again youtube casting is possible, but not screen mirroring (TV doesn't appear as an option).
My chromecast appears as options for all types of casting on in both locations from all relevant devices.
As to Windows 10, I don't believe any desktop PC can screen mirror. I think the mirror function is looking for an active wireless adapter to project to the network. I've looked at the Project function on two of my desktop PC, and neither show the option to project to any networked display. But my Windows 10 laptop sees everything on my network, not just Roku devices. But it does see both of my Ultra players (one is wired and the other is wireless), my two Roku TV (again one wired and the other WiFi) as well as my two Shield players. I have tested projecting to my Ultra 4640 and it worked fine. At that time, the laptop was connected to my network on a 5 GHz access point, but it's also worked when connected to the same access point on the 2.4 GHz radio.
I also checked my work laptop this morning. Even though it has a wireless adapter, I'm using a docking station and a wired network connection. It also doesn't show the ability to see other networked devices, so that seems to validate the wireless adapter needs to be in use.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.