use 5.11 for SNMP, as 5.10 received SNMP changes.
Here's an update:
This morning, I connected to the other brand-new MikroNOC 8400 with 5.2, with the intent of upgrading this one to 5.11 in hopes that would solve the SNMP problem. Just out of curiosity, before I installed the upgrade, I tried to do an snmpwalk on it, figuring it would not work, and giving me a decent baseline. Nope - this one doesn't have the problem.
I made sure that the SNMP settings (as shown by export) are identical between the two routers, and the only firewall filter rule that appears at all relevant is identical (action=accept chain=input comment=UDP disabled=no protocol=udp).
I can pass an snmpwalk request through the problem router to a device on the other side, and it works - so that also suggests this isn't a firewall blocking issue.
So now, I can still do the upgrade, but it looks as though the RouterOS version 5.2 is not the sole cause of the problem, and I am not confident that the newer OS will solve it. I feel like I am missing the important key to the puzzle, and the solution is just beyond my reach. Any ideas?
set contact=ecpi enabled=yes engine-id="" location=Austin trap-target=0.0.0.0 \
set xxxx-xxxx address=0.0.0.0/0 authentication-password="" \
authentication-protocol=MD5 encryption-password="" encryption-protocol=\
DES name=xxxx-xxxx read-access=yes security=none write-access=no