Whenever you reach a new city, you need to download the respective content (such as the relevant maps and missions) to be able to access it.

If you are unable to download this content of the game, or you get stuck during the process, this may indicate an unstable internet connection or lack of memory on your device. As a first step, please try following the intructions below:

1) Free up storage space by deleting unused apps and media on your device.
2) Follow the troubleshooting steps outlined in this article.