I've just started noticing this as we've added all of our PMP450 AP's to cnMaestro finally. Random PMP450 AP's will show as being offline in cnMaestro, however I know they're online, and cnMaestro even shows their clients online as well. When I visit the AP's in question, there's nothing in the event logs showing issues, and the status always shows that it's connected to the cloud with the proper ID.
There is no known issues with 15.1.1 , to analyze further , Could you please send engineering.cgi output to firstname.lastname@example.org.
If you are seeing what I've seen from the very start, is that the device is "offline" to cnMaestro - Not offline as in "STA_DROP". The latter being a true offline condition. These "offline" status events were so bad at the beginning that I moved to onpremise, though I just noticed cloud version is at r18 and on premise is at r8. This eliminated the alert for the APs and has greatly minimized, but not eliminated the alerts where the SMs are concerned. The APs also used to give DNS failures when trying to resolve cloud.cambiumnetworks.com, but would resolved everything else. Since the use of onpremise eliminates the DNS lookup... whalah! There are still some "offline" alerts on the SM side, but have been greatly reduced. I don't know how many PING/Pong handshakes are allowed to fail before the alert is raised. The case where I was able to verify an actual traffic loss at the SM have been extremely rare.
If you can, try using onpremise, I think you will be pleasantly surprised. If you are currently using onpremise, then you are having a problem different than the one I was having.
Regarding your question on SM management when in NAT mode, you must use "Standalone" mode.
Hope this helps.
Root cause for techdump not getting downloading when SM is in NAT mode with remote management interface configured as WAN has been identified and fix will be available in upcoming 15.2 release. For now as workaround use standalone interface for remote management