You can sync calendar services using iCalendar feeds. If you are trying to connect your Teamup Calendar to a different calendar via outbound iCalendar feeds and it's not working, here are a few things to check:
Make sure you are using an iCalendar feed URL, not a calendar link.
A calendar access link, whether it's a shareable link or an account user link, will not work as an iCalendar feed URL.
To get the iCalendar feed URL for your Teamup calendar, open it in a browser and go to Preferences > iCalendar Feeds.
Make sure you are in a browser. The Teamup app does not provide access to iCalendar feed URLs.
Does the calendar have iCalendar feeds enabled? If the option for iCalendar Feeds is greyed out in the Preferences menu, it means that the option is disabled. Contact your calendar administrator to enable this option.
If you have successfully set up an outbound iCal feed and it stopped syncing, see these troubleshooting steps.