The Hologram SIM is setup to permanently roam on our partner networks and holds the credentials necessary to access 550+ networks worldwide. That being said, the choice to roam and connect to a network is largely driven by the cellular modem and firmware which controls when and how devices connect.
Traditionally a modem will scan all available networks and connect to the one that provides the strongest connection. Network availability depends on the bands your device can access, its geographical location, tower traffic, and a few other smaller factors. Ultimately, however, this boils down to your device's firmware, so if you have any doubts we recommend contacting your manufacturer or visiting our general firmware advice guide.