If the sulfa doesn't work may I suggest you buy some Baytril? Baytril will treat many of the bacteria that can cause respiratory infections. This article list place where you can buy it.
https://www.backyardchickens.com/articles/baytril-enrofloxacin-sources.74374/
https://www.backyardchickens.com/articles/baytril-enrofloxacin-sources.74374/