Two routers, two subnets with static routes. Need help

Hi guys,
long ago i asked for help for my first openwrt config ( Help for first config ever with OpenWrt - Installing and Using OpenWrt - OpenWrt Forum, and with your help i solved my problem and all is working as i need.
Now i need i little more help, here there's a simple recap:

1.One net managed by FritzBox 7530 (modem/router with ppoe passthought), on this net i have an Home Assistant server, several IoT devices, and smartphones
2. The second net is managed by a netgear router with openwrt, on this net i have 3 set top box (sky q platinum and 2 sky q minis) and my main desktop pc. On this router there's a sqm setted up to avoid bufferbloat on my pc when someone is watching a streaming service on the sky box.

Before i split the networks, i was able to cast youtube and use spotify from my smartphones to the sky boxes, and now i can't. I suppose since the smartphones and the set top box are in two separates subnets. Why i can correctly use the sky boxes with my home assistant server (server on the fritz, sky box on the netgear) but i can't cast? Is another service that is not covered by the static route (that i setted up with your help in the previous 3d)? Can i do something to make it work?

Does this thread help? Chromecast across two subnets - #13 by joe_internet