If you ever get authentication failures when trying to log on to NetScaler Gateway with credentials you know are correct then start logging the authentication attempts on NetScaler using aaad.debug to find out what is going wrong.
netscaler
Citrix Insight Services
Citrix Insight Services (CIS), formerly known as Tools as a Service (TaaS) is a set of tools available online focused on making the support of your Citrix environment easier. CIS does this by collecting information on your environment, analysing that data and responding with recommendations for you to make to that environment such as hotfix patching recommendations or configurational changes.
Configuring LDAPS on Citrix NetScaler
If you want to enable LDAP Secure for NetScaler authentication follow the below guide. Using LDAPS allows you to use the Allow password change option on NetScaler so Active Directory users can change their expired passwords. For this reason, and the security advantage, many people opt in to using LDAPS with NetScaler.