Recommend ways to make Wi-Fi faster?

In the house we have at&t internet and my nefew (10yr old) who comes home from school at 2:45pm plays video games till 5-6pm and then again at 8-9pm till when ever he goes to sleep. I mean he is way to much into video games so much I have seen him all time playing video game on Nintendo switch and watching videos on YouTube at the same time. (Once he was on three electronics at one like wth). But anyways he takes up all the Wi-Fi and makes my phone slow for my internet (I have iPhone XR) what can I do to have better Wi-Fi?
Even if he is just on his xbox it takes up a lot of Wi-Fi.
And No it's not other ppl on there phone that makes are Wi-Fi slow just saying

Limit his time. You can set controls on the Wi-Fi Router to only allow his PC access during certain hours. Playing games that long is not good for his development.

You can block his gadgets or slow him down. It's all in the router settings.

If you're the one paying for the Internet service then you get to control access to it. However, other than blocking a device via its MAC address, I haven't had an AT&T Wi-Fi router yet that had the capability to limit access on a time-of-day basis. Which means you'd have a little bit of setup to do at the beginning then you'd have to manually enable and disable the MAC filter whenever you wanted to change things.

List your router's make and model (usually on a sticker on the bottom of the device) which will give us a chance to look up a copy of the User Guide for you.

Buy different router.

Turn off the xbox and kick nephew outside while you enjoy internet. Or, true evil way, buy new router, give him the slow access and you keep the fast one (2 band Wi-Fi) and he can be on all day/night and still can't get faster internet than you, buahahaha.