I think these models have incomplete support and need more work.
In the debug, it’s hitting the bottom block (line 837):
SNMP['/usr/bin/snmpget' '-v2c' '-c' 'COMMUNITY' '-Ovq' '-M' '/opt/librenms/mibs:/opt/librenms/mibs/sonicwall' 'udp:HOSTNAME:161' '.1.0.3.0.6.0.1.0.2.1.3.0']
So what do you get from .1.0.3.0.6.0.1.0.2.1.3.0 - presuming nothing relevant?
You’re getting what looks like valid data on .1.3.6.1.4.1.8741.1.3.1.3.0 but the code as it’s written won’t use that as it expects the sysObjectID to be .1.3.6.1.4.1.8471.1 in LibreNMS/OS/Sonicwall.php
Will need others to tell us if they see this on their devices also and what other models do, would potentially need the logic to be altered in the code to account for it. For example in your case, checking the logic to match for .1.3.6.1.4.1.8741.1 -or- .1.0.3.0.6.0.1.0 would get it working.
I suspect from this, that the device doesn’t play by the original rules: Sonicwall Gen 7 OS - #3 by SegaMegaDave
The PR which responds to the above mentions CPU is not covered: Add Sonicwall OS 7 by edgetho007 · Pull Request #12997 · librenms/librenms · GitHub