In addition to an iPhone 6 (or later) with the latest iOS, you will of course need an Apple Series 3 with GPS + Cellular. In addition, Apple requires that you have completed a suitable mobile tariff for the Apple Watch. Make sure, however, that iPhone and Apple Watch use the same provider. In Germany, only Telekom and Vodafone currently support the eSIM of the Apple Watch. In addition, you should be in the reception area during the set up.
Setting up mobile on the Apple Watch Series 3 (GPS + Cellular)
When you pair your Apple Watch Series 3 (GPS + Cellular) with your iPhone for the first time, you will be guided straight through the setup process You also set up the mobile connection. Alternatively, you can also subsequently start the configuration.
To do this, open the Watch app on your iPhone and open the tab "My Clock". Then tap the "Mobile Network" option and then select "Configure Mobile". Then follow the step-by-step instructions to set up the tariff. The process may vary from provider to provider. If you have problems, contact your wireless service provider directly.
By the way: To save power, the Apple Watch does not always rely on its own mobile connection. Once your iPhone or a known Wi-Fi network is nearby, the data connection will be established.