Sorry for the delay in posting back.
I have sorted this issue.
I installed the SCADA on a different PC, which has a dedicated connection to the VPN and no other internet connection.
Also, I just realized that I have to add 1 to the address when doing MODBUS Reads.
E.g. [PSS_Tank]1.HR164 is what I wrote to read my register address 164 on the device.
However it needed to be [PSS_Tank]1.HR165.
Thank you all for your support.
Shenal