I know how frustrating it can be when a system does now work as advertised. As someone who has been in the computer field for over 33 years, I know how the sales dept can make promises that the tech dept can't deliver on.
Anyway, here are a few things you may want to try before throwing in the towel. I'm going to assume that the mobile device is an iPhone but if not, the same basic steps should apply.
Logout of your the Verizon Home Monitoring & Control App.
Uninstall the app.
Close all open apps.
Wait a minute or so (really, count to 100) and reboot the device. On the iPhone, this is done by pressing and holding both the "Home Button" and the "Hold Button" at the same time until the Apple logo appears on the screen, then release the buttons. This will really clear the device and make it ready for a new copy of the app.
Note:The closing of all apps and rebooting of the iPhone is something I do every night before I go to bed. I don't care what anyone says, it make for a better running iPhone.
Now, reinstall the Verizon Home Monitoring & Control App but don't open the app just yet.
This next step is optional. Reboot your the Verizon Home Monitoring & Control gateway device. I do this by cutting the power to the gateway. I have a my gateway next to my Verizon router in the attic. The reason for this is so I get better coverage throughout my home and around my property. Since the exterior walls of my home are very thick (it's an old house), I get better coverage going through the wooden roof. And to save trips to the attic, I have my gateway on a remote power switch (WalMart, $11.95). Shut down the gateway for a minute or so (really, count to 100 again), then reconnect the power. Let the gateway cycle through for a few minutes (go make a sandwich).
Once the gateway is ready, open the Verizon Home Monitoring & Control App and login.
You'll know if the gateway hasn't finished booting if you get your favorite error, " Service not available at this time". Just give it a few minutes and if it is still not working, try rebooting it again.
If all your device appear on your app, you should be good to go.
Hopefully, you won't have to go to such extremes again. But all advance systems will start getting a little screwy if they run long enough and it helps to reboot every now and then.
Good luck and please let us all know if this works for you.