Connecting external DB

I found the answer.
for those who may experience the same issue in the future I’ll leave a note here

when initiating auth0 login page in your app, it should point to custom DB that was set up in auth0 rather than default Username-Password-Authentication

   var authorizationUrl = client.BuildAuthorizationUrl()
              .WithResponseType(AuthorizationResponseType.Code)
              .WithClient(clientId)
              .WithConnection("YOUR-CUSTOM-DB-CONNECTION-NOT-Username-Password-Authentication")
              .WithRedirectUrl(root + redirectUri)
              .WithScope("openid offline_access profile email")
              .Build();