Subway complaint: Violations of ADA

I have a medical condition for which I use the assistance of a medical dog. This dog has been serving me for 9 years and has traveled the world with me through my work. He also accompanies me to doctor’s appts, restaurants, my work office and any other place that I go.
Today I went into the Subway located at 6000 S. Western St., Amarillo, TX, which I have visited many times in the past, and was told that I could not come into the place of business unless I had paperwork proving that my dog was a trained service do.
They did not ask the ONE question that is Legal according to the 1990 Americans with Disabilities Act (What service does your dog provide) but instead discriminated against me and my service animal by refusing service and requesting qualifying papers.
To me, this is akin to demanding to see someone’s paperwork to prove they are legal to be in the country. This, however, has also been declared to be illegal.