Do you have any external server available to you? If yes you could use that to set up a VPN connection or SSH tunnel.

Tailscale is probably your best option.

This seems like it might be a perfect use case for Tailscale. The open source version of the control plane is called Headscale but you’d have to host it in a vps somewhere if you wanted to use it.

I use headscale and headscaleui but I’ve heard things about net bird and netmaker

frp, or just anything on awesome-tunnel at GitHub list

I use zerotier. It’s excellent.

Cloudflare Tunnels are black magic and exactly what you’re looking for:

https://developers.cloudflare.com/cloudflare-one/connections/connect-networks/

Free, no need to self host a server somewhere externally. Can even be used for SSH!

If you are fine with ipv6, I’d use yggdrasil.

Just install it on any number of devices and each of them become part of a single «local» ipv6 network.

Here’s a big list

OpenVPN