OW causes my whole network to slow down immensely

Hi! I’m having a lot of issues with my internet regarding Overwatch lately. I’ve always had a very inconsistent connection but I’m very used to that. My problem is that as of recently I’m not only lagging on Overwatch, I’m also causing every device and application on my network to slow down to the point where it barely works. It also causes Discord to lag (closing Discord doesn’t help). The lag is so bad that OW is unplayable. I used to be able to play Overwatch only at night because that’s the only time I’m the only one on the network, but now even then I can’t play.
I am using wifi and I’m going to buy an ethernet cable soon, but I’m wondering if there’s anything I can do until then. My ISP isn’t trying to help me and I’m unable to get a new one.
I’m also not able to directly connect my router to my computer due to my mother’s work’s policy.