nonbinary person refusing to correctly gender a trans woman
classic tme behavior. I think a good idea would be to have someone else talk to this person about things: it sounds like they won’t listen to you but they might listen to someone else. If that doesn’t work I would probably try going to a higher-up (even if that’s something like a shift manager). You’ve said you’ve talked to them four times: that’s honestly being assertive. The final thing I might recommend is literally having you/someone else correct them every single time they get it wrong, which would be exhausting but miiiight eventually get through to them? It’s been a while since I’ve dealt with this kind of problem, and I’ve found that either people will get it or you just won’t get through to them and you’ll have to put up with it or get rid of them somehow.
addendum since I didn’t actually say it in my comment: I hate to say it but it might be more useful if the person who’s advocating for you isn’t a trans woman. it’ll hopefully be fine even if not but this person sounds like an absolute asshole: I’d be happy to slap them for you.