Searching for Networks Manually or Manual Roaming for iPhone/iPad

1. Tap Settings > Carrier and turn off Automatic. Wait until available networks appear (this can take up to two minutes).
2. Tap the carrier you wish to use.
3. Return to the main Settings screen. 
   

Note: You can perform the above steps in case you lost signal on your phone suddenly (ex. while roaming). This helps your phone find network manually.