“90 Day Fiancé” star Armando Niedermeier-Rubio is unable to enter the country to be by his daughter’s side as she’s being treated in the ICU … because TMZ has learned he was turned away at the U.S.-Mexico border.

Here’s the deal … Armando announced Wednesday his daughter, Hannah, was rushed to the ER at a Mexican hospital after half her face became “droopy” and half her body lost its strength. Assuming something was wrong with her brain, he and the doctor decided to transfer Hannah to the USA so she could obtain better care.

Armando’s husband, Kenny Niedermeier, tells TMZ … Armando tried to enter the country using a letter from the hospital, but he was turned away.

We’re told Armando is still in the process of obtaining a spousal visa.

Hannah’s not totally alone … Kenny says her grandma and aunt with her at the Arizona hospital where she’s receiving care … and he’s on his way after cutting short a trip home to Toledo, Ohio for his brother-in-law’s funeral.