My dog is coughing for a very long time and no medicine is helping

Hi,
I adopted a 3 months old chihuahua mix few months back. The puppy came to my home and was coughing. Went to vet and with they did the exam and the conclusion was Kennel Cough.
An antibiotic was prescribed. After that was done the cough was still there. We went to the vet again and between that time and now different antibiotics were prescribed (from 3 different vets):
Clavamox
Clindamycin
Some other ones that I don't remember the name.

Before the current one the Vet did a culture of the sample collected from the throat. As a result the Doxycline was prescribed. The result was Ecoli. That is probably not the cause of the cough, that may be there because she still eating stool (we are working to stop that).

The last vet I went did a XRay and seems like everything is fine but the lungs. The lungs show some white dots that, based on the Dr diagnostics, are related with bronchitis.
The doctor prescribed for that the following medicines:

• Doxycycline
• Cefpodoxime

We are giving those two medicines for 6 days already and I didn't notice any change. I don't know what to do anymore...

During the day she does not cough that much, very rare. It happens at nigh, that she cough here and there but in the morning (5AM) when we wake up is when she cough the most, she cough a lot for few minutes and after that start to reduce and she goes well during the day...

Any simmilar experience that could help our puppy? She have been getting antibiotics one after the other and I want to avoid it...