If you are having trouble with your managers , call your towns board of health. If a customer says they have an illness that precludes them from wearing a mask, you cannot ask what their illness is, but I can tell Yu that I would add to them that it's unsafe for them especially to be out, and if it's no mask no service - that's how it is.
ADA outlines say like, "you have a right to be served to the restaurants best ability and at no risk to the employees" in as many words.