I knew it had to be something on my computer because I could connect fine on my phone via wifi - so not the internet itself, not my ISP, not my router, etc.

I think my mum might be having the same problems, as she mentioned something odd to me the other day. I'll check it for her. But for me, the bonjour service said it had last updated about mid June, which corresponds with problems I've been having. It's been an issue for other people for years, though, according to my google search.