All rats are carriers for myco I think unless they are born in a myco-free environment in a lab, unfortunately (which is usually via c-section, according to what I've read).
Have you gotten a decongestant/expectorant from your vet? In my experience, a decongestant/expectorant with antibiotics is key and it saved my rescue rat's lives (they are still alive).
There's no good in medicating a rat without it since the fluid and mucus will not be removed by the antibiotic, nor will it help the rat remove it if it's really bad. Fluid and congestion will only breed bacteria, and like others here have said already, dirty bedding will irritate the airways, causing more mucus and fluid to build up.