Please check if you can Powershell Test-NetConnect to the device on the port KEPServerEX is trying to use.
Powershell has a built in commandlet that can help prove or disprove TCP socket connectivity
Command Syntax: Test-NetConnection [host] -Port [port number]
Example: Test-NetConnection 255.255.255.255 -Port 502
If these do not succeed this issue is networking.
I would recommend you to start with security as "None" and verify OPC UA Client configuration also as per below article.
https://www.ptc.com/en/support/article/CS298913
If you are prompted to log in to view the complete article, you can use your MyKepware login credentials.