Oh, I understand that - trouble is the configuration at these sites is subject to change & I don’t have direct access to their routers & firewalls. No trouble opening ports, but assigning a static wireless address, port forwarding to it, etc would not thrill their IT department x 100 sites.
My other option is to set up a small PC (Raspberry or other) running Windows and a remote software - VNC or Teamviewer or other. I prefer the thought of using routers, I’ve had uneven experiences with DDNS. The autodiscovery of these routers behind firewalls gave me hope, but won’t work for me because I need to have the other end of the VPN in a software client.
If we think this is just too difficult to set up, I can accept that, Technically it should be feasible.