IPhone AOL calendar sync

When I reinstalled caldav to sync the AOL calendar to my phone, it doesn’t work. However, it continues to work on my iPad. Does anyone know how caldav is supposed to be installed and configured on iPhone 6s? Also, does anyone know how to port all of the calendar history from the AOL calendar to something with a good product life expectancy? I have more than 20 years of calendar data on AOL that I would like to retrieve and retain. Suggestions? Am I toast?