How to remotely connect Macs across different networks?
Assuming you have a central and stationary management unit (a Mac) and several remote Macs (1:n relationship), I would choose an inverted VPN-approach:


Set up a VPN-server on your central Mac. The network address of the VPN-net mustn't collide with local network addresses. Port-forward all necessary ports to your Mac on your local router. Depending on your Internet service provider and your Internet connection get a dynamic DNS-address.
Set up VPN-clients on the remote Macs and connect them to your central VPN-server. The clients mustn't route all traffic via the VPN-connection.
Enable Screen Sharing on the clients
Connect "locally" with your VNC-app.




Since the question misses some details (e.g. macOS versions/number of remote Macs/safety rules in the remote networks/24-7 service/Internet &amp; network stability locally and remote etc.), I can't answer the question in all its particulars.