I’ll make a couple assumptions, correct me if I am wrong.
Your PLC used to be on a subnet that the Ignition server could route to. Maybe the same subnet as it is currently, but either part of a larger vlan or just untagged all together.
Your PLC is now on a vlan tagged subnet.
Your Ignition servers IP hasn’t changed, nor has it had an additional NIC added to it on the new VLAN subnet.
Assumptions over.
In order for Ignition to talk to this PLC, you’ll need something like
Ignition to be on the vlan(single NIC) or have a NIC on that vlan, OR have its nic capable of VLAN tagging and allow it to have multiple virtual IPs(one of which should be on the same vlan)
Ignition’s IP added to the VLAN acl to allow it to route to the PLC and vice versa.