You do not need a static or public IP at the remote site to establish a VPN connection as a client. A cheap contract, a cheap SIM card with only data transmission should work, no need to ask for a unique or static IP.
On the other hand, the internet connection in the plant where the SCADA is needs to have a static and public IP but this is easier to achieve for a cable or ADSL connection.
If you still don't understand it yet, consult a network administrator.
It is the same system that many people currently use to work from home or from remote sites.