When adding LRE Host, you may see below error in LRE Server log:
2020-04-06 15:12:34,496   ERROR WcfBridge.WcfCaller - Error sending WCF request [Context: (null)] [FullContext: (null)]
WcfBridge.WcfCommunicationException: Failed to communicate with the remote WCF service. Connection Info: ILabService
Domain: DEFAULT, Project: LAB_PROJECT, User: xxxxxxxxx.
The issue occurs because LRE Host cannot communicate with LRE backend service over port 5876 (default port)
Staying on LRE Server and LRE Host, try to access http://LREServer:5876 and make sure you can access it, if not make the port 5876 is open from LRE Host to LRE Server