Bandwidth controller

[Erundil]

Hi everyone, it's my first post here i'll explain my issue

I live in Italy and internet connection are really bad here, i have something like 7,3Mb with a 20Mb contract but whaever, point is there are a lot of devices now connected to the home network since my family went on a "tech spree" and brought everything that they saw that had a screen and a wifi connection, so basically we have like 4 phones, 2 tablets and 1 pc connected to the WiFi and my pc connected via cable, that's not a big deal since all they do is browse random news sites and such, problem is when my little sister watches youtube or someone connects with their iphone after being away, i guess it starts syncing everything with iCloud and internet literally dies.

i wanted to ask is there a way to limit the amout of bandwidth the wifi can use? like let's say i set it to 1Mb for them so i'm not forced to totally shut off the wifi when i want to play? i already tried QoS, but the main modem i have doesn't have that option, so i bought a new one but it doesn't work with the internet line for some reason, and costumer support of my IPS told me they can't help me with other modems, so i'm currently using that Asus modem as an access point wifi, while my pc is connected with cable to the main modem.

I've done a lot of research, i even asked a friend that works in networking but he's really busy and can't quite solve my problem, so this is my last hope lol

thanks to everyone that will try to help me