I am currently researching the same thing as we have 2 English Bulldogs and they can't handle heat at all.
I am looking at something like this camera/temp sensor on Amazon
It seems it would work for us since I usually set up a cellular hotspot whenever we camp for my work and for TV streaming.
Another option is the Marcell device
It uses its own 4G cellular connection, much more expensive, and only offers temp sensing, no camera.
Good luck and please do let us know what you decide and why as it may help me decide too. Thanks.