Please make sure your WiFi signal is stable. If you are using mobile data instead, you may have to manually allow the application to use your phone's mobile data. Please keep in mind that you cannot play the game without your phone being connected to the internet.
We have included some troubleshooting steps from Google Play and Apple to help you out with connectivity issues:
Google Play:
https://support.google.com/googleplay/answer/2651367?hl=en
Apple: