I’m afraid if I am active in the Union or contact the Union requesting help regarding a problem I am facing my supervisor will treat me differently or things will get worse…

You should know that Management cannot discriminate against you in any way whatsoever for your affiliation with, or participation in the Union or for your filing a complaint against them. If a manager has directly told you the “right thing to do” is not to be active in the Union or not contact the Union for help — this is potentially ILLEGAL and may constitute harassment, abuse of authority or a combination of both, all PROHIBITED CONDUCT within the United Nations. If this happens, we need to hear about it. Being active within the Union or seeking help is your business only, not your supervisor’s.