Hello,

little bit confused by your description and requirements. my idea, pls check if I understand this functionality ok.

You need manually set ON Modbus functionality and roles on each devices, correct IP and Ports are required due expected network. Then you can use from PLC Modbus TCP protocol to connect each required Server (device) over network and set via documentation required relay. Basicaly,  I don't see any problem in case all is described inside the manual (Modbus commands section about relay control).