I use Alfred on my old Android devices. Not sure if all functionality works on Apple?
When the device is unplugged (loses power) I get an alert. It also allows me to watch/listen and talk through the monitoring device. I do recommend using a cellphone/tablet with built in flash. This allows night vision to work better. It can be set to detect and alert to motion. Works great on whatever spare smart devices are laying around. Uses your home wifi. If wifi is unpredictable or not available you could use Google Fi and the Nexus 5x still on sale.
I did connect a very old Motorola Droid X. When the camera focuses from the rising sun it sets a motion alert. The Samsung Galaxy S3 does not have this problem. You could setup a Bluetooth temperature alert device or place a thermometer within eyesight of the spare smartphone/tablet.
New Pay as you go phones can be bought for near $100 if you look around. You will want the monitoring smartphone to be connected to power. It will burn through battery power. The loss of power alert would be ineffective unless plugged in.
A couple of months ago someone posted on this forum some Bluetooth temp alert devices that connected to a spare smart device. I tried to convince the wife the dog needed her own active smartphone. Apparently that's excessive. I since discovered the Alfred app using existing tech around the house. I still wouldn't mind a Project Fi Nexus phone for the dog.
Google Play Store Alfred