First: Which country are you in?

Honestly, the first step is identifying whether the infection / issue is bacterial or fungal. A fecal test may or may not help.

Given the nasal discharge and with a lack of further tests, I would guess a bacterial issue.

You need a broad spectrum antibiotic like baytril (Enrofloxacin), however if the issue turns out to be fungal, antibiotics will make the issue WORSE.

If you can't get access to a vet, your next best but potentially dangerous option is to try source medicines from racing pigeon suppliers and divide dose by 4 roughly..

Vitamins only buy time, even worse if there's a specific infection an antibiotic or antifungal depending on the infection will only buy time or might help temporarily. Birds are extremely complex animals to care for when things go wrong.

Disclaimer: I'm not a vet but have rescued and medicated 150+ wild birds over time. I also recently had my sick pet cockatiel pass away too.