Is there no network on your mobile phone when you are abroad? Change your mobile phone’s settings and check whether your device is adapted to the local network.
Normally your mobile will automatically connect to another available operator as soon as you are abroad. Is this not the case, or do you prefer another foreign network? Then change the network settings of your device manually.
Other network frequencies are used in some parts of the world: 900 and 1800 MHz in Europe, 1900 MHz in North America and 850 MHz in South America and Asia. Almost all models these days are “multi-band” and switch from one frequency to another without you even noticing. But if you have an old phone, you may not be able to use it in some countries and regions. You can check the available frequencies on the manufacturer’s website.