I don’t understand your problem. Please provide information as described in PLEASE READ BEFORE POSTING: What’s required to diagnose a problem.
I have a (legacy) FastMail family account, with the following calendar situation:
4 calendars created/owned by my user
1 external public calendar subscribed via ics link
1 calendar shared from the masteruser
1 calendar shared from other family member
Using the “Login with email address” option, DAVdroid detects the 4 calendars owned by my user and the subscribed calendar, but not the shared calendars. When I contacted FastMail about this, they predictably but unhelpfully said it must be a problem in the way DAVdroid was detecting.
After playing around a bit, I was able to reach the two shared calendars, but somewhat messily.
FastMail provides a CalDAV URL for every calendar, configured as: https://caldav.fastmail.com/dav/calendars/user/[username of calendar creator/owner]/[long string of alpha-numeric characters separated by hyphens]/.
If I add a DAVdroid account with the “Login with URL and user name” option, using the FastMail url for one of the shared calendars, my own FastMail username, and the 3rd-party app password, the newly created account has that shared calendar AND all the calendars owned by my user plus the one ics subscribed calendar (but not the second shared calendar).
I can then go through the same process with the next shared calendar and get a third DAVdroid account that contains the second shared calendar AND all the calendars owned by my user plus the one ics subscribed calendar (but not the first shared calendar).
I’m able to achieve synching all my calendars without duplates by selecting only the shared calendar for synchronization in each of the two additional accounts, but I’d love to be able to streamline this all into just one account, if possible.
And, if that isn’t possible, at least this information might help someone else hoping to sync a FastMail shared calendar.