Hi noiz,
Normally, NTM can't discover nodes that doesn't respond to SNMP request. To check if your device responds to SNMP try this:
1. Click Help -> Support tools -> Grab SNMPWALK.
2. Put the IP address of the said router and the correct Community string.
3. Click Scan and make sure that you can download OID information.
If you receive an error, let us know.