System Center Configuration Manager 1511 and Below.
Device icon indicates - Online Status Unknown:
A computer is considered online if it is connected to it's assigned management point. To indicate that the computer is online, the client sends ping-like messages to the management point. If the management point doesn't receive a message after 5 minutes, the client is considered offline.
Make sure SCCM server has TCP port 10123 allowed on incoming.
To check if client status will display as "Online" you may set Heartbeat discovery and Client status message to 1 so it will be checked every day.
Additionally, in Assets and Compliance > Devices > right-click on the columns bar and add Devices Online Status column to check if the status has been confirmed.