In order to pair a smart device to a personal hotspot you will require a 2nd phone. This 2nd phone will provide a wifi network to your phone, the smart device will be paired to this network. This is called personal hotspot on iPhone or mobile tethering on Android.
To do this, using phone 2 navigate to Settings, tap on Wifi and then turn on the personal hotspot or mobile tethering. You will see a password is displayed.
On your phone, go to settings and then wifi, tap on phone 2's hotspot network from the available list. You will then need to enter their hotspot password.
Once connected you are now using their internet.
Get the smart device into pairing mode again and re-add it to the app, this time you will enter the hotspot password when prompted for the wifi password.
So long as you are near the smart device with both phones and the password is entered correctly, the smart device should pair.