Check if service-discovery is working: visit with a browser https://cloud.myserver.com/.well-known/caldav
This should redirect you to the webdav interface and after entering user credentials you should see “This is the WebDAV interface. It can only be accessed by WebDAV clients such as the Nextcloud desktop sync client.”
If this is working, do that on the iOS device:
Settings -> “accounts+passwords” -> “add account” -> “other” -> “add caldav account”
As server enter only the domain name nothing else (in your example this would be “cloud.myserver.com”). Also enter username and password for the nextcloud user and hit “proceed” in the upper right corner.
With working service-discovera iOS will automatically discover the proper url. Letsencrypt is no problem for iOS - it works here without any issues for the last year or so.
Good luck!