I'm so sorry to hear that. Yes, same as the others have said -back to the vet. They may be able to give pain killers which should stop her hurting enough for her to chew it. But it's obviously causing her pain which is why she's trying to chew it off. I'm not sure if they do it because of infection, I think it's just because it hurts and they want to get rid of the thing that's hurting. If the painkillers don't work then they may need to amputate her leg. I saw a hamster owner on YouTube recently had a similar problem & that was the route they went down. (Unfortunately, her hamster later had to be euthanised as he wasn't having a good quality of life but not because he was 3 legged.)
I'm not sure Serendipity that I'd want to wrestle a "cone of shame" on to a hamster, even if their anatomy allowed for it! I quite like my fingers as they are
But their pouches go back too far and they can slink out too flat with their bendy spines to have any hope of one staying on.
But yep, the only thing to be done, I'd say is the vet again. And if that's no help, see if their is one near you a bit more specialised in small animals. I hope it works out okay x