Hello,
I have been using any device on the local network for a long time, from wherever I need it, and the system I use is completely transparent to routers or gateways.
It is very simple, and free.
The only requirement is to be able to install the application on the device that needs to be connected.
In the case of Raspberry it can be done, and also, of course, on Windows, Mac, Linux. Even some routers, but then you need to understand more about routers.
What I do is register an account on Zerotier, and there create a vpn network.
With the network codes that we have created, the zerotier client is installed on the devices that need to be connected to each other.
This vpn acts as a second network card and the zerotier cloud acts as a virtual switch.
The result is that you have an independent network that has its own IP range, and that is only accessible from another node on this network.
You have all the ports open without having to open them on the router. And since it is a good VPN, everything is encrypted over the internet.
I use it to, for example, connect from a PC to my icom 7300 remotely, from any bar, accessing the internet from my phone.
If you need more information, don't hesitate to ask me.
EB3AM Jordi