Ok as you then have the latest eM Client V9.2.2307 and the NextCloud Calendar url works fine on your Android mobile, then you will need eM Client support to further investigate that.
So if you have a current active paid Pro or Personal version, I would go to the VIP support page and login and lodge a support ticket.
Ps I and many other eM Client users found a similar thing with the eg: Yahoo CalDav Calendar where it added in perfectly into the accounts area, but just “very rarely ever appeared” as per my post in the following thread with V9.2.2307.
I then tested adding the Yahoo CalDav account into Outlook on Mac and it appeared perfectly and never dissapeared as per my post in this thread. So Microsoft were doing something different to get it to work. Unknown.
So due to the dissapearing Yahoo Calendar in V9.2.2307 , I personally ended up transferring all my Yahoo Calendar events to my Google and Microsoft account Calendars in eM Client.
So if you have an alt Google or Microsoft account, till your NextCloud Calendar is working, I can only suggest to maybe use one of those alt calendars in the meantime.
Lastly if there is other NextCloud users on this forum that have managed to get the Calendar to work in V9.2.2307, they will no doubt update this thread.