I am trying to add a calendar in iCal format to my phone.

Fossify Calendar is nice and fully offline, using DAVx5 as sync adapter.

The link looks like

https://example.com/cal/export/5o2b2j393b21owbd829273b3

In DAVx5 I can only add an account, which does not work for this link. The only direct link to add is a WebDAV filesystem mount

Solved

You need this app for ics links that are unidirectional. Annoying (another app running in the background) but works.

Same with DAVx5, after adding the calendar, go to settings in Fossify Calendar and “add CalDAV calendars”.