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.

Tailscale is probably your best option.

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

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

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!

I use zerotier. It's excellent.

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

OpenVPN

I use tail scale to get on the LAN and Rustdesk for desktops, webuis or SSH once in.

zrok

Yggdrasil Network might be an alternative.

It's a breeze to set up, just be mindful about setting up your firewall and allowed keys, depending on your use case.

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.

If you already own a domain name, you could use Cloudflare tunnels using cloudflared (it's free).