Manually Roam to different networks
The below steps will allow you to manually roam to a specific network when using a multi-network SIM
Method for iPhones
Go to Settings and then Carrier (or Mobile Data -> Network
Selection for newer versions of iPhone)
Toggle OFF the automatic mode and manually change the
carrier.
Leave the menu and wait until it connects.
Method for Android
Go to Settings and then Wireless networks.
Go to Mobile networks and then Network operators.
Search for networks.
Select a network
Wait until it connects.