The right way is to go to your Namecheap account, click on manage next to your domain. In the domains tab, navigate to NameServers section and you need to select Custom DNS on the main page and add the NameServers.

Use the two NameServers used at FirePress in the dotted lines and click on the green tick to save the changes.

Avoid this mistake!
When you go to the Advanced DNS section on Namecheap you can only add NameServers for a subdomain and not the bare domain.

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