No, you won’t have to do anything from the TrueNAS shell.
In the TrueNAS GUI, Go to System → Services, and click on the pencil for SNMP.
From there, click on SNMPv3 support. The SNMP user is managed on the right side of that screen.
Set the user up for DES under Privacy protocol, and save, and then restart the SNMP service.
Make sure that your SNMP client is set up to use DES as the SNMPv3 privacy protocol, and see if you can reach SNMP that way.
I should have posted this link:
That shows the other user’s original setup.