Your Sony Xperia Z5 automatically joins the fastest data connection available. In some situations, you may want to switch to 2G, 3G or 4G. To manually select the connection type, follow these steps.
Note: Switching down to either 2G or 3G respectively when in either a 2G only or 3G only coverage area will help save battery life.
To start using a faster internet connection again, switch back to a higher network mode.
Go to the Start screen.
Choose Apps.
Choose Settings.
Choose More.
Choose Mobile networks.
Choose Preferred network type.
Choose GSM only.
The Sony Xperia Z5 has been set to the lowest possible network mode.