Try this, email an .ics file to yourself and then attempt to add it to the Calendar app on your iPhone or iPad. Does it work? It didn't for me as of 2021-07-31. Seems like it used to work. A web search suggests that other people are having the same problem. Possibly for security reasons Apple turned this feature off?
Anyway, this has an impact on a FileMaker Go demo database I made a while back that exports .ics files. Recently a reader, Nellie, wrote to say: