Expose server under NAT

Avoid router port forwarding.

Serveo

nano /etc/systemd/system/serveo.service

Paste:

[Unit]
Description=Serveo
After=network-online.target

[Service]
User=root
ExecStart=/usr/bin/ssh -R dtoservice.serveo.net:80:localhost:3000 serveo.net
#ExecStart=/usr/bin/ssh -R dtoservice.serveo.net:35080:localhost:3000 serveo.net
RestartSec=3
Restart=always

[Install]
WantedBy=multi-user.target

Enable service:

systemctl daemon-reload
systemctl enable serveo
systemctl restart serveo