My server was deleted from ranking when I wanted to migrate the order of my old server.

Old server: 37.148.210.9:9987 (rank 17.)
New server: 185.30.167.50 deleted

Okay, first of all I really don't understand how the exact same domain pointed to 2 different IP addresses and how it has 2 different ranks here for the same port. Clearly some tracking delays here but I might be wrong.

Maybe GameTracker is going over capacity tracking all those bot activity within fake servers while still being de-ranked. Your old server is still being tracked in it's old rank linked below, so do whatever you trying to do wisely:

Old server: https://www.gametracker.com/server_info/37.148.210.9:9987/
New server: https://www.gametracker.com/server_info/185.30.167.50:9987/

You posted to me that this is your new server: https://www.gametracker.com/server_info/4code.net:9987/

It is currently showing as on line and scanning.

I don't see the problem here.

However, at no point in our instructions How do I migrate player stats or rank between server pages? does it suggest that you should add a second server.

If you are switching to dns, simply update the original server page to use your new dns. It is that easy.