The 3G Internet service in Cuba is available for prepaid Cubacel lines 😏 But you need to know some things about it:
To activate Internet Data in Cuba...
You must use a cell phone able to connect to the 3G 900Hz band (Phones sold by foreign telcos should check particularly this detail: companies usually restrict the spectrum of phone bands)
You also need to activate the Nauta APN. Users in Cuba who use Nauta mail already have it activated. (Instructions for Android, and for iOS). If you don't use the Nauta mailing service, activate the APN and use the 3G network to access a free site such as etecsa.cu or mi.cubacel.net
How to activate Cubacel's 3G Internet?
If you meet the two previous requirements, you are closer to being able to use Whatsapp in Cuba 😉
Just dial *133#
In the menu, choose the "Datos" option. You'll see the two options to activate the Internet: Paquetes de Datos (Data packages) and Tarifa por Consumo (Pay as you go rates).
Any of these allows you to use messaging apps and surf the Internet, but it is good to keep in mind that the purchase of data packages is much cheaper.