I'm sorry I prol do not have much advice.
What I would do, presented with your problem (with the limited knowledge I also have with nets.. ) is.. make sure the system you are interested in, is on the proper SUB NET(S).
This is where I would *start*. If that did not help.. Then.. (while waiting for answers
.. I would start over in setting up my whole net.. making sure that your original problem (of "..able to connect to the PLC as though it sat on the 172.16.254.xxx..") was sorted FIRST, before I accomplished the balance of the set up.