There are 2 reasons for devices to go in to "Unknown" Category in OpManager:
1)Device credentials (SNMP/WMI/CLI) are not added or discovered with wrong credentials in OpManager.
2)Credentials are correct but OpManager does not have built-in Device Templates.
1)Add the credentials to the Credential Library , Go to settings > Discovery > Credentials > Add Credential>select the credential type based on the device type > provide the credentials > Save and then try to discover or add the device using the credentials.
2)To create a new Device Template > Settings > Monitoring > Device Template > Add Template > type in the Template name based on the Device Model (ex: Cisco 2960 Series ) , Vendor ( ex: Cisco , if you do not the see the vendor in the drop down list , create new and add it) , select the Category (Ex: Switch) , click on Query Device , provide the device IP or name ,type in SNMP community > click on Query , this will get the SysOID from the device and add that to the template > Save the Template , now try to discover or add the device using correct credentials which will categorize them properly.
After the above steps , devices will be categorized properly, however if the device is matched with the above new template , we need to add custom SNMP monitors.
Refer the link below to add custom SNMP Monitors :