You need a DNS A record for discourse.guidekeeper.com pointing to the IP address of your server.
Nameservers are generally provided by the company that you purchased the domain name from, not Digital Ocean. So you likely want to change it using the DNS control panel at the place you purchased the domain name from.
In the event that you are using Digital Ocean’s nameservers (but why?) then you would need to change it using the DNS control panel at Digital Ocean.