"Home Hub not connected"

I use an AppleTV (gen 4) as a Home Hub for my HomeKit devices. I have a lot of devices and automations and recently they're not working right. As soon as I leave my home wifi, all of my devices are "not responding". This means my automations don't run. Which means my house doesn't get locked. This is obviously a major problem, particularly because I do not get any sort of notification or alert that there is a problem, which means my house is unlocked all day and I don't know it. NOT OK. When I am home, everything works fine, which makes it very difficult to troubleshoot.

But I did just notice one thing and am hoping it might be a key- in the Home app settings it says my Hub is in "Standby". Not "connected". Standby. It says this when I am away, and when I am home. And I just noticed that even though I am home now and can control my devices, all of my automations are not working because it says 'home hub not connected'. So maybe things are just working without the hub now? i.e. I don't need the hub since I'm home and on the same network, so I don't notice it's not working?

Why is my home hub not connected, and how do I get it to connect? I went into the settings on the appletv and everything appears to be fine. ios/macos/tvos are all updated to the latest.