The app has three functions: Mac9 for accepting preplans, Trip scanning, and a voice feedback system that I think is absolutely worthless. Obviously, the Mac9 and the Trip scanning are the only two functions we use on a regular basis, and even the Mac9 is occasionally as I usually use the Qualcomm. Both of these functions require either a cellular or wifi connection to work, the Mac9 to receive the preplan info and send your acceptance or denial, the Trip scan to send the scanned documents. The Trip scan feature requires a camera function on whatever device you are using. I see no reason why a tablet wouldn't not work as long as it had a camera and wifi capabilities. An external scanner will not work.
Also, it's not necessary to have this app. As I've said before, concerning the Mac9, I usually use the Qualcomm to accept my loads. The only time I may use the app is when I'm at home or maybe sitting inside at a truck stop eating and can't run out to the truck. And you can scan your trip bills at any location with Transflo or at the terminals. So if your phone doesn't support the app, I wouldn't worry about it too much. You don't really need it.