mail server – domain name system – rDNS does not exist

I need help with Reverse DNS because I have a problem with that. I have installed rDNS on my hosting provider that I use rdns1.mydomain.com and I have a server host name that is gateway1.mydomain.com. I want to use two different names to separate it, because the mail providers want to see the two different names so that my email is sent to the inbox.

When I open the Email Deliverability software via Cpanel, I can see this:

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Can you please advise me with what I need to do?

How can I correspond with the rDNS?

Should I change the host name of the server to match the rDNS or what?

I would like to use smtp513.mydomain.com to send e-mail and use gateway1.mydomain.com to access whm and cpanel. I'm not sure what to do, so I need your help for that.