I have been 'avoiding' this one in the hope that someone with a better answer would jump in.... but, since they haven't..... here is my faded memory cells version of how it went......
Back in my Factory Floor days, I did exactly this.... But I also found what you have found (controller resetting etc).
The M4SENET-100 card is both a blessing and a curse.
It is cool that we can take an older controller and have it connect to something like Ethernet, but it is also a tease as it really is pretty hard to use as a more modern connectivity option, which is what it seems to offer.
The best way that I can describe it is that Ethernet has the throughput of a fire hydrant, but the LCM4 has the throughput of a drinking straw.
Thus what you are looking to do has (to borrow from the upcoming Olympic games) a difficulty factor of 15 out of 10.
The key to getting it even close to working is to monitor very carefully every session. DO NOT close it after every transaction.
The TCP/IP standard means that the session has to linger after its shut. It does not go away straight away. This is not something that Opto can do anything about. It is part of the way the stuff works.
If you close it after each transaction, thinking you are doing the right thing in cleaning up after yourself, you will fill up all your sessions and even Opto Control will not be able to connect (Nor Opto Display, nor Opto Term etc.).
The telnet session into the M4SENET card will become your best friend. You will be real cozy with it and how to refresh the connections list (which is gold as you can see the seconds remaining for each connection, both open and closed).
That said, you obviously have to detect when a session is orphaned asap. The longer you leave it open, the longer you will not not have that session available down the track.
Taking a step back, I have to ask, why not just upgrade to an S2 controller?
It will do exactly what your asking with ease, and still talk to your Misitic I/O.......
No really, an upgrade in this case may well be quicker and less painful. This coming from a guy that has done both....
Not super helpful I know, but that's about the best I have. Sadly the chart I ended up writing is long long gone in the 'archiving' of many a hard disk drive since the event.... I just clearly remember the 'pain' it caused (and its the only time I have e-v-e-r come really close to bricking an Opto controller (had to put it in fail safe loader mode to get it back)).
Of course, your mileage may vary as tools like wireshark and the core of understanding about all things TCP/IP has had ~12 odd years to mature since I was last trying said maneuver.