I misunderstood this originally, but the problem here is that a virtual server is for DNAT and it seems like what you actually need is SNAT.
With the setup you have now, lets say your machine is 10.15.70.100.
When the traffic from your machine hits the router at 10.15.70.19:502, what happens is the router sees a packet going from
10.15.70.100 --> 10.15.70.19:502
changes that to
10.15.70.100 --> 192.168.1.88:502
and sends it on its merry way.
When the PLC receives that, it is going to be trying to respond to 10.15.70.100, but because it doesn't have a default gateway it can't do that.
If this PLC is connected to the same network as your PC normally, then as @joeqwerty said you could just use a secondary IP address in 192.168.1.0/24 and everything would be fine.
If you want to do it this way with the router and NAT, then all you'd have to do is reverse the router ports, and add a route on your PC to use the router to get to the PLC.
Make it so that the LAN port is on the 10.15.70.0/24 network, and the WAN port is on 192.168.1.0/24, and create a route on your machine saying 192.168.1.0/24 via 10.15.70.<router interface>
With the other settings on the router at pretty much default, what will happen in this case is the router will see:
10.15.70.100 --> 192.168.1.88
change that to
192.168.1.<router address> --> 192.168.1.88
and send it along.
Then when the PLC wants to respond, it can respond to the router at 192.168.1.x which it will be fully capable of doing.
