If it is automatically taken, you cannot change the mapping with the context menu on the inventory/topology view ("set device mapping"). It will be seen as fully recognized - see attachment
If you can change it to different OIDs, the device isn't fully recognized.
The OID you will find with
snmpwalk -v1 -c <communityname> <ip> system
Run this from the FortiNAC to ensure the snmp connectivity to the device.
If you cannot poll the device, you want to see what errors you have there saying that you cannot poll and what exactly you are testing.
Like pushing the Poll button obviously, or the automatic poll throws up error logs in the event logs of FortiNAC.