Easiest way if the PLCs are ethernet based is to create a dial-up networking connection, using either a SCADA PC with a modem or an ethernet modem such as N-TRON 102RAS.
If security is an issue, the customer can always control the connection of the phone line to the modem.
I had one customer years ago , who had a two position switch in his office, one position connected the phone line to the SCADA PC modem, the other position to the company's retail store's Debit Machine dial up line.
Yeah a remote modem, VNC Viewer running on the SCADA client which runs e.g. simatic manager, this will allow remote control of that station but tends to be a bit slow but would do the job!