Help with constant ping spike

So I'm using a campus' internet and the internet is connected to a modem/router. The main problem with my internet is that whenever a 2nd or 3rd device connects to the wi-fi my own wi-fi gets totaly wrecked. Almost everytime by download speed decreases by 80% and my ping alternates between 20 ms and 5000 ms. Also to make it clear there is nothing wrong with the campus' internet one of my friend uses it and he has 0 problems. What would you suggest to solve this problem???

Which router and openwrt version?