If you have you purchased your domain with another registrar but want to host your website with Pay Monthly Hosting you will need to amend the NameServers to point to "our" servers.
Ask your Registrar to amend the NameServer records to those detailed in your New Account Information email.
The instructions for setting NameServers at some of the common domain registrars are here:
Godaddy
123-reg
Bluehost
Hostgator
Ionos
Namesco