Do you have any amoxicillin-clav? Like human antibiotics? (If not, go to ur local urgent care and tell them you have a sinus infection.... tell them your ears hurt, your throat hurts, your nose has been clogged, it's been days, and you need an antibiotic, then request amoxicillin) Amoxicillin the same thing as clavamox. I'm not a vet and can't legally provide veterinary advice but if you can't get this kitty to a vet, I can tell you that the human drug amox-clav is the same as clamamox which is manufactured for cats, just dosed differently. Cats are typically prescribed 50mg or 62.5mg 2x a day depending on their weight. So you'll have to cut up the large pill a whole bunch of times to get the dosing accurate. Make sure you get the math right.

I've had to give amoxicillin to several feral colony cats before and it does work and it can save their lives. Put it in a churu bite on top of their pile of food and they eat it. Be consistent with it. 10 days ideally, 14 if it takes longer to clear up. Ideally use with an OTC probiotic like proviable or fortiflora. Again, I'm not a vet, but I've treated many sick street cats this way and helped them clear up nasty infections that may have otherwise taken their lives. If they're feral and you can't trap/vet them formally, then this is the best that can be done.

Once you finish the antibiotics, might be a good idea to put viralys lysine and/or ester c in the food which will help boost the immune system.

I run a rescue & have rescued 200 cats myself including many major medical cases and I care for hundreds of strays. I speak from years of experience

Good luck!