We have a Doorbird in place at a gated entry. It's locally powered and connected back to the house LAN via fiber bridge. The first unit would not connect to LAN and was eventually replaced under warranty. Apparently early DB units had problems with the pigtail wiring detaching from the main board--DB confirmed this was the problem with our unit.

The replacement unit worked flawlessly for months, then began not showing up on the app. Oddly, the homeowners would call and say the Doorbird was offline when monitored by their LAN-connected phones and/or tablet, but I could see it and operate it remotely with no problems via DB's cloudserver. The frequency of the drop-outs increased, so we recently replaced the unit and began experiencing the same intermittent issues over the span of 3-4 weeks. Then the issues cleared and the DB has been stable [knocking] since...

In the process of troubleshooting, DB tech support noted that a customer-provided photo of the iBezel docked and LAN-connected iPad that monitors the DB all the time was indicating connection to the DB via the cloud [which makes no sense, given they're on the same LAN]. When we asked how this is possible, they replied that the DB provides three streams. The first two are assigned to the first two LAN clients and the third feeds the DB cloud server. All clients beyond the first two connect via the cloud. As such, if the cloud is having issues [which DB denied] and/or there are DNS or service provider problems, some local devices may not be able to connect.

I agree with others' assessment that there's something up with DB's ap and/or cloud server.