How do I do this…multiple networks

So I have AT&T Fiber internet. The modem they provide is a modem/router combo. My house is wired with Cat5e which works well, but the Wi-Fi from the AT&T router absolutely blows, my solution is google nest. I have the modem/router under the main TV, Ethernet goes out to the primary Nest router, PS4, AppleTV and AT&T TV. From the Nest router, it goes out to the wall, travels upstairs and out to a nest access point. All of this works pretty well, I get the correct speeds on every device, my wifi feels pretty good throughout the house.

Here’s the problem. I often need to use my phone as the apple remote (children lose that remote all the time). This works GREAT, as long as the phone & appleTV are on the same network. The problem is, with the phone on Wi-Fi and the appleTV hardwired, they aren’t on the same network. If I use Wi-Fi for the appleTV, I tend to get bandwidth & buffering issues, so I prefer to have it hard wired.

What do I need to do to let my phone on Wi-Fi communicate with the appleTV thats hardwired direct to the modem/gateway?

Thanks in advance. I’m usually good at figuring stuff like this out, I just don’t really know how to approach this one and its been a long time since I really messed with any networking stuff.