DNS and mail server question

Hi, I followed many tutorials but still cannot figure out something about my DNS and Mail servers.

First in registrar:
My domain themecook.com comes from Namecheap, I registered two nameservers ns1.themecook.com and ns2.themecook.com
Each of them is pointing to a different IP that I have set up with my VPS host on the same box though.
Then I assigned themecook.com to the same ns1.themecook.com and ns2.themecook.com.

In Virtualmin:
I created a new nameserver named themecook.com and applied it the default template

I also created 2 new “A - IPv4 Address” records for ns1.themecook.com and ns2.themecook.com.

The site worked for a while and after trying to add other NS records (ns1 and ns2) it is not showing anymore.
I also created users for mail but they keep bouncing when I write to them.

I have no clue if I should remove the default hostname chosen by Virtualmin, it is my VPS hostname called vps74898 and why sometimes it is vps74898.ovh.net.
IntoDNS tells me ns1.themecook.com and ns2 are not setup on my host.