crontab @reboot question

Hi everyone
I'm a new user here and to Linux
and here goes my first question

I have recently installed KDE plasma on my Kali, and it's great
but I have an issue that it reset its screen res to default every time I log in
I have tried different way to go about fixing this, but it didn't work
it doesn't bother me
I've made an alias for it using xrandr, and I change it every time in a sec
but what I don't understand is why my @reboot command in crontab don't work

this is my code
@reboot xrandr -s 5
and I made another code using my alias
@reboot res
and it says installing new crontab or whatever in my terminal
but neither work and nothing happens at reboot

What am I missing here?

Thanks guys