Why doesn't the app show an IP address for a connected device?
If you designate the IP address on the device itself, you will not be able to see an IP address in the Plume app, even if the device is connected to the network and working. This is considered normal behavior, but maybe unexpected.
The reason for this behavior is that the device is not actually making a DHCP request, though it already has an IP address that was assigned to the device itself.
What devices is this app behavior common to?
This type of device setup is common to devices that require a specific IP, such as IP security cameras. The device software itself will either already have a specific IP assigned to the device or will force the user to assign an IP. This is not the same as an IP reservation being set up on the router, although this step will also be required.