Monitoring agent software must be installed to monitor a system in an agent mode. Usually monitoring starts within minutes after successful setup. You can wait forever if something is wrong.
This time no network connection issue occurs. There are entries inside OperationsManager Event Log explaining OM Agent behavior.
NT Event 7017 is well described with error explanation and remediation suggestion.
HSLockdown tool confirms LocalSystem account has no access to Management Group – Deny is always “stronger” than Allow.
Using this tool (/R switch to remove an entry) you can change permissions by removing denial.
Changes in permissions must be followed with HealthService restart to be effective.
Agent icon turns green as a result of described actions.
This case shows one of many possibilities of “grey agent” problem. Except for network access check, you can examine OperationsManager Event Log to find an actual reason.