External monitor not working after changing DVMT

I was trying my external 4k monitor on my dell xps 13 9360 with Mojave, and it was working but animations were slow, like open launchpad or switching desktops. So i tried to increase DVMT from 128 to 192, but it didnt solve the problem, indeed when i disconnect the monitor, the internal display stay black. So i returned to set DVMT to 128, now when i connect the 4k monitor with the display port, is recognized but it says "no signal from display port" and then to sleep mode. If i use duplicate display mode it works, and so if i connect the display with HDMI.

I tried to search on google ways to reset (?) or dissociate (?) monitor and i tried to delete this files:

rm -f /Library/Preferences/com.apple.window*

rm -f /Users/user/Library/Preferences/ByHost/com.apple.window*

rm -f /Users/user/Library/Preferences/ByHost/com.apple.pref*

but it didnt work.

If i reinstall Mojave i think probably resolve it, but i'd like to know why after changing DVMT settings the display doesn't receive signal anymore.

Thanks

submitted by /u/n0-where
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