Android does not support CalDAV or CardDAV, which are used to sync calendars and contacts. However, a workaround is installing a CalDAV or CardDAV sync adapter.
We have tested and recommend DAVx⁵, which is approximately $5. Once you have added your account in DAVx⁵, you can set up calendars or contacts in the app of your choice, and the changes will sync with Fastmail.
https://www.fastmail.help/hc/en-us/articles/1500000279881-Se...