My 4G won't work!

I checked my plan online yesterday, and it said "10GB of data shared between 3 phones", and showed my cellphone number as well as my mom's and my sister's. However, my mom is the only one able to use 4G. It will simply not show up or work on both mine and my sister's phones. I have both "cellular data" and "LTE" switched on under my settings, and wifi turned off - still no 4G. My sister and I were just recently added to the data plan and have not been able to connect to the 4G since the change. I have the iPhone 5, and she has the iPhone 4S. Help! 😕