I have a wireless system set up for my home that does not depend on the ISP's router. I plug a lan port of the ISP's router into the WAN port of my router. It works fine. That way I can use whatever modem I get from the ISP and my home network doesn't need to change. I should go in the ISP's router settings and configure that device to be in bridge mode but I haven't yet.
At the front of my house, I have another wireless router set up in the same way, the lan port of the main network is plugged into the wan port of another router. This inner network is used to control my holiday (halloween and christmas) light shows. It allows the controllers on the show network to see the internet if needed but keeps network traffic between my home and my show separate. I could have set up a subnet but I chose this route instead. The network in a network works fine for my purposes here. This seems to be what you're looking to create. This setup does not isolate the things on that network from the internet.
My BI computer has 2 NICs. One is plugged into the home network, the other into the camera network. This does isolate the cameras from the internet.
I recently picked up a couple of travel routers. I have them set up to run wireguard, similar to openvpn. One is a client and the other is the server. When I travel, I can plug the server anywhere into the network. When I'm at my destination, I can connect the client router via cable, wireless repeater, or tethering. When I connect to the client router with my laptop, I can see my home network. My main network is a mesh network and doesn't have a vpn option. If it did, I would not need the travel router that plugs into the home network.