SonicWall SNMP: connection timeout

Hi. I'd like to monitor a SonicWall firewall via SNMP. I configured the service as per SonicWall KB, but when querying the firewall via snmpget I just get a timeout:

# snmpget -v 2c -c public 10.2.2.1 1.3.6.1.2.1.31
Timeout: No Response from 10.2.2.1.

otherwise if I use alphabetical oid:

# snmpget -v 2c -c public 10.2.2.1 sonicFirmwareVersion
sonicFirmwareVersion: Unknown Object Identifier (Sub-id not found: (top) - sonicFirmwareVersion)

so this looks like a reply, negative but a server reply.

I checked the firewall rules and everything seems in place (probably automatically configured when enabling SNMP on the interface). Why am I getting timeouts?

It's aTZ 600, firmwareSonicOS Enhanced 6.2.7.1-23n.

thanks