Wifi relay for specific subnets

I have a router for a TV that has a cable that goes out to internet. I want to add to it that whenever it gets a call to a specific subnet it would act as wifi relay to my other aka main wifi router.
Main wifi router has media attached to it and I would like to share it to the TV over Wifi but all the other time have it use wired connection.

If I make my router a relay it relays all traffic via wifi and I do not need that.
Here is the guide I was following.

How do to modify this config to work only for specific subnet?

P.S. Using Wifi antena on my tv router only for it to be a client and not share wifi is fine/preferable.

I'm having difficulty understanding your situation. Why not use wired all the time?

Can you draw a diagram of your physical topology? That may make it more clear what you are trying to do.

Also, if you want to have streaming type services all use one subnet while you have a media server on another, why not simply create firewall rules to allow the TV to access the media server via routing across the subnets? This works easily unless there are same-subnet type requirements such as mdns or other auto-discovery methods. In that case, you can use an MDNS repeater/reflector to handle the cross-VLAN zero-conf requirements.

You could also use Policy Based Routing to have certain types of traffic directed to certain routes (for example, streaming services go out via a VPN, etc.).

Here is my architecture I am looking to add the dashed line between router over wifi.

I can no use wired LAN connection because of physical obstacles..

Firewall rule for my specific external IPs? That should work. I was trying to avoid opening any public ports but it sounds like a workable option.

I tried running zerotier but the router behind TV can only handle around 2-3Mbps decoding AES, so it is way to slow for most HD media.

You don’t need to open any public ports at all for normal internet access. And with policy routing, it should be possible to send the local traffic where you want.

Maybe wireguard would be a better option??