There are two routers opeating in my house, the first is ASUS RT-87U, which is the primary router, and the second is D-Link DIR model, which is the secondary router. It is connected to the primary router through an Ethernet cable (Primary: LAN Port 2, Secondary: WAN Port).
The reason I have a secondary router is because I use it for VPN connections, ie. ExpressVPN (L2TP Protocol).
My problem is that I want both routers to be on the same subnet. Ideally, if I wanted to cast something through chromecast (assuming chromecast is connected to a different router that the one my device is connected to), I should be able to.
If I switch to a LAN-LAN setup, I would not be able to connect to the VPN server, which is why I have this router operating in the first place.
I hope my language is clear, and have not mislead anyone.