CRM for Outlook server supports two connection methods. One is through the CRM for Outlook Cloud Connector and the other is direct LAN connections. The advantages of local LAN connections from clients are many including speed, stability, and lower data usage. CRM for Outlook clients will by default try to connect over the local LAN, but to do so they need to be able to access the CRM for Outlook server on a specific port. To ensure local LAN connections work go to the server GUI and click "Advanced Settings":
Then check the port number listed:
and make sure this port is accessible and opened in the firewall for LAN connections (and VPN connections if you wish to support those). Once done only an initial check is made to the CRM for Outlook Cloud Connector on startup before the client shifts to LAN connection only.
To check the connection type on the client you can go to "Settings":
See also this video: How to open ports for direct LAN Connection