Two Routers + HomeKit + Apple TV

Hi everyone,

I have a rather specific HomeKit setup and am looking for some OpenWRT config advice to get it working fully, please. I've spent hours on it so some success. I'll explain below but first here is my network scenario:

Primary network: ISP/modem router generating a mesh Wi-Fi network (using Plume technology) — works beautifully. All of my HomeKit hubs (Homepods) and HomeKit devices are on this network.

Secondary network: a Linksys WRT3200 running OpenWRT connected by ethernet to LAN port on primary network. Only devices connected to this are Apple TVs. The reason for the network is that its the only reliable way to watch streaming services in another country via VPN...this works extremely well. The VPN client being turned off or not does not change the outcome of the problem explained below.

The problem: I recently installed a Starling Home Hub to my primary network allowing Nest devices to work in Homekit. This works very well. The issue initially was that I couldn't use Homekit via Apple TV (cameras, device switches, etc.) at all via the secondary network. I learned that Apple TV does not access Homekit remotely like other Apple devices do. Only via local network.

I was able to get Homekit working generally via the Apple TVs, running on the secondary network, through a combination of port forwarding, traffic rules in OpenWRT and Avahi. I'm not sure Avahi was actually needed but regardless it's currently running. I can turn on and off lights and that sort of thing.

So currently the only issue is that I can't streaming video on the cameras on the Apple TVs via the secondary network. The rest works well. The developer of Starling kindly tells me that the video streaming runs on a randomized port in the range of 32000-65535 via UDP and it changes each time.

So the question is, how to configure my OpenWRT firewall and port forwarding to allow this to actually work.

Thank you in advance and let me know what other information I can provide!

A bit of a clarification that was a long explanation I posted. I just need to allow flow of traffic between the Starling Home Hub and my Apple TV over local network(s).