In the US, we currently work on AT&T and T-Mobile. While this can be manually chosen, by default we'll select the network work with the best reception in most devices.
To manually choose a network on mobile phones, used the Cellular settings to choose a Network. Be sure to have the correct APN in order to connect.
To manually choose a network on a 4G/LTE modem, you man need to reach some advanced setting in your devices. Contact support for additional help.