When your domain is registered with any other registrar, but you need to host the site on RealHOST hosting, you need to change the NS of the domain to RealHOST name servers:

• ns1.realhost.pro
• ns2.realhost.pro
• ns3.realhost.pro

These changes should be done at your domain registrar.

You can find out who is your domain registrar, for example, here - https://www.whois.com/whois/