Before you try to connect to a group calendar, you should make sure that you’ve set-up your server correctly and that you can securely access the web gui using a web browser from your iOS device.
Yes, I get a https connection to the login screen for nextcloud, so I think that’s all good. Not sure if this matters but I have nextcloud installed on a server in my garage, using Open Media Vault and Docker.
It doesn’t matter where you’ve installed Nextcloud but how you access it, over wifi or mobile network etc. Additionally you need to make sure that you can always access it using the defined dns name and not e.g. the ip address.