Connecting PLC and Internet modem to one PC

I have a PLC connected (TCP/IP Modbus) to a PC running SCADA in windows XP SP2, I want to remote that PC from the PC office. My question is how can I connect a modem to the port RJ47 that is already taken. should I use a switch or add another internet card to the PC. Do I have make some configuration like setting port forwarding so that the computer deals with both, PLC network and modem internet. I am still beginner in this domain

In general, you should install a second Network Interface Card (NIC) with its own Ethernet port, which will be configured to connect to the Internet or to your enterprise network.

Windows XP SP2 is very old, and may be difficult to configure for both a limited-purpose LAN and connection to your enterprise network or the Internet.

If you have an IT professional available to you at your facility, it would be ideal to ask him or her to examine the computer's networking setup to be sure it will perform both functions.

Are both pc's on the same network? If so, you could use something like Remote Desktop. Windows XP comes with Remote Desktop if I remember it's not difficult to set up just follow the help file.