I have my nextcloud running on a normal Webhosting from domainfactory.
Used owncloud before and switched to nextcloud recently. Haven’t tried the iOS app though. Decided that I rarely want to have a look at files from my phone and if I do neee access to some files (calendar or my org-mode to do list) I do it via WebDAV
You need a certificate. This is a problem that iOS introduced in one of their recent updates. Everything has to be SSL encrypted now. I had to get an SSL certificate for my nextcloud subdomain. That should fix the issue.