I recently had some Modbus/TCP server boxes that were installed in 2018 fail to talk to the Windows based SCADA after a corporate network upgrade.
It turned out that the server box vendor had run into this situation previously. Apparently, newer network management software uses cyber security protocols that shut down Modbus communications, in part due to non-Modbus open ports on the servers. The vendor had a firmware upgrade for the server boxes that fixed the problem, along an updated with a Viewer app package that failed after the network upgrade, too. The SCADA package worked once the servers were firmware updated but the SCADA package got an update anyhow. It was at the version level it was at in 2018. The vendor took 5 months to come up with the upgrade. I'm glad my customer wasn't the first to experience the problem and that the firmware update was already available.
Unfortunately the vendor is very closed mouth about details about why the old firmware didn't work and what the network security protocols were that closed down the Modbus traffic.
So you might ask if the network has had a major 'upgrade' coincident with this failure to communicate.