6to4 tunnel not working on ubuntu servers

Question

I have two servers (both ubuntu 22.04), one doesn't have a public IPv6. I'm trying to create a local connection between them using 6to4 tunnel (they have eachothers v4 ping).

I've created /etc/netplan/local-tunnel.yaml file on both servers with the following details:

on server1:

network:
  version: 2
  tunnels:
    tunel01:
      mode: sit
      local: server1-v4
      remote: server2-v4
      addresses:
        - 2a01:4f8:1c1b:219b:b1::2/64
      mtu: 1500

server2:

network:
  version: 2
  tunnels:
    tunel01:
      mode: sit
      local: server2-v4
      remote: server1-v4
      addresses:
        - 2a01:4f8:1c1b:219b:b1::1/64
      mtu: 1500

and put the following inside /etc/systemd/network/tun0.network

server1:

[Network]
Address=2a01:4f8:1c1b:219b:b1::2/64
Gateway=2a01:4f8:1c1b:219b:b1::1

server2:

[Network]
Address=2a01:4f8:1c1b:219b:b1::1/64
Gateway=2a01:4f8:1c1b:219b:b1::2

then executed netplan apply and systemctl restart systemd-networkd to apply the changes.

The problem is that I cannot ping server1-v6 from server2 and vice versa (I can ping local v6 of each server, showing it is configured, but there is no connection between them via v6)