I finally gave up on this. We use PRTG to monitor our entire network. For a while I thought my PRTG Windows 7 machine was the problem. When PRTG lost access to the Mimosa radios, the whole Windows 7 machine lost access to them (could not ping from command line, could not get web page to open up for any Mimosa radio).
I then installed a second PRTG install on a Windows 10 machine on a different physical part of our network. Guess what?? After two days, all Mimosa radios stopped responding to SNMP requests from THAT server as well for exactly one hour. Again, can't ping Mimosa radios or access them via HTTP from that Windows machine.
So, I now have two different computers on two different parts of our LAN that both can't get at Mimosa hardware for exactly one hour each - but at different times of the day. When they can't talk to Mimosa radios, they are the ONLY devices on the network that can't access them.
It really looks like the Mimosa radios have a firewall or something in the software that shuts off access from a certain IP every 24 hours for 60 minutes. It's like it thinks it is seeing an attack and stops access. I can prove it is not my hardware and not my network with the problem. What also makes me think this is in the Mimosa software is the time of day it shuts off access directly correlates to the last reboot time of the monitoring machine. So, if our PRTG server was re-booted at 3am, at 3am two days later, Mimosa radios stop allowing access from that PRTG machine for 60 minutes.
It's hard to explain in words but I really think there is something in the Mimosa software that makes it "protect" the radio by black listing an IP for 60 minutes.