not able to add a cisco 4221 router as snmp failing

Port 161 is for snmp communication while 162 is for snmp traps. Two different functions.

Have you tried doing a packet capture from the solarwinds server to the device to see if it's properly replying? Have you checked to see if the access lists are correct? Are there any firewalls blocking access? Have you checked your device snmp configuration on your cisco? Not snmp traps, but rather the SNMP settings itself. Have you confirmed that the community string is correct? These are just a few of things to check. And could lead you to where the problem might be.

I  hope this helps.