Identity Server not returning refresh token

You do have to explicitly ask for 'offline_access' in your request. Separate the other scopes you are requesting with a space. (In my examples below I am replacing 'Default' with 'MyApi' to be clear that we are talking about a scope defined by your app.)

&scope=MyApi offline_access 

However, you must also grant that client the right to get refresh tokens, it doesn't just happen based on the flow you pick:

var client = new Client()
{
    ... //All the stuff you were doing before

    ScopeRestrictions = new List<string>
    { 
        "MyApi",
        StandardScopes.OfflineAccess.Name, //"offline_access" -for refresh tokens
        //Other commonly requested scopes:
        //StandardScopes.OpenId.Name, //"openid"
        //StandardScopes.Email.Name,  //"email"

    },
}

You may need to add 'offline_access' to your scope store as well. The scope store is the list of scopes that Identity Server knows about. Your question doesn't mention how your scope store is set up in your project, so you may already have it. But if the above doesn't immediately work for you, you may want to look around for code like this in the example you're working from and add OfflineAccess.

var scopeStore = new InMemoryScopeStore(new Scope[]{
    StandardScopes.OpenId,
    StandardScopes.Profile,
    StandardScopes.Email,
    StandardScopes.OfflineAccess,  //<--- ensure this is here to allow refresh tokens
    new Scope{
        Enabled = true,
        Name = "MyApi"
    },
}

Add offline_access value in scope while sending token request

new Client
            {
                ClientId = "ro.angular",
                AllowedGrantTypes = GrantTypes.ResourceOwnerPassword,

                ClientSecrets =
                {
                    new Secret("secret".Sha256())
                },
                AllowedScopes = {
                    IdentityServerConstants.StandardScopes.OpenId,
                    IdentityServerConstants.StandardScopes.Profile,
                    IdentityServerConstants.StandardScopes.Email,
                    IdentityServerConstants.StandardScopes.Address,
                    "api1",
                    IdentityServerConstants.StandardScopes.OfflineAccess
                },
                AllowOfflineAccess = true,
                RefreshTokenUsage = TokenUsage.ReUse,
                RefreshTokenExpiration = TokenExpiration.Sliding

            }