If your SQL server is in domain and if you are using default SQL database instance, then the SQL user would be a domain user.
DP server is not in domain and hence the user also will not be in domain. So you can not add DP user to domain without server joining the domain. So you should make the DP Server Join the domain.
After this, configure the inet service in the services to use the SQL user you have configured which also must be a DP user.
Noramally I install anything after the machines are in domain and hence these problems do not arise, except sometime need to configure users in security of SQL.