No zone file on the side of your Custom DNS

This is the intended behaviour. We don’t use cPanel. So of course the behaviour is different to cPanel’s.

However, the online file manager is down these days. So you must use desktop FTP to transfer your files.

Sadly, that’s already the simplest guide we can provide to you. This is web hosting, not Microsoft Word. It’s not like “I enter some words in a random place and everything works”.
So please try read it again. And, if you failed to understand, don’t say “I can’t understand, give me simpler guides”. Tell us which part you are failing to understand so we can better help you.