Just:
cargo build
or cargo build --release

Start server by command:
./server <port>

Client start up command is:
sudo ./client ws:https://<server_ip>:<port>

Sudo is required because it's create a network interface vpn with a name: vpn0.

Next step is to asssign ip addresses to the clients:

# client 1
sudo ip address add 10.0.2.2/24 dev vpn0
sudo ip link set dev vpn0 up

# client 2
sudo ip address add 10.0.2.3/24 dev vpn0
sudo ip link set dev vpn0 up

That is enough to connect two devices via vpn via websockets.

# from client 1
ping 10.0.2.3

# from client 2
ping 10.0.2.2

Requirements:

nodejs and npm

Let's create simple html page on the client 2 (10.0.2.3):
echo "test vpn" >> web/index.html

and then expose it via simple http server from node:
npx serve -l 54321 web/

So, right now should be possible to curl from client 1 to client2 on port 54321 and receive content of the index.html file.
curl http:https://10.0.2.3:54321

Let's assume - client 1 want to access to the internal network of client2 (10.0.0.0/24).
To do this client 1 needs to set up routing 10.0.0.0/24 subnet via 10.0.2.1 (vpn) gateway:
sudo ip route add 10.0.0.0/24 via 10.0.2.1 dev vpn0

client 2 should forward packets from vpn0 interface to the internal network interface:
sudo iptables -A FORWARD -i vpn0 -j ACCEPT;sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE