There's a bad odor?
If so, then it may be Infectious Coryza. Do you notice any lesions or canker (yellow/white pasty material) inside their beaks?
If it's infectious Coryza, then you can try a Sulfa antibiotic. Smz-Tmp would be a good choice, also Baytril (enrofloxacin) which is a strong antibiotic can be given too.
I'm sorry they are not improving a lot. It does sound like they have secondary infection that needs to be treated with a different antibiotic beside the Tylan.
It's good that you are getting testing. Let me know what you find out.
Here's some info where to find the other meds and dosing.
https://www.backyardchickens.com/threads/lethargic-and-weak-light-brahma-hen.1591172/post-27038025