Welcome to the Dell Community  @Nikoz 

Since you installed Ubuntu 18.04 maybe it would be better for you to post your question here???

https://ubuntuforums.org/index.php?s=b4361b21262ffa42fc260aabb0398877

As the people on that forum deal with Ubuntu questions more than here.

Best regards,

U2

hi , i had a similar issue and it got fixed after i followe d teh steps as in the foloowing youtube link ,

https://www.youtube.com/watch?v=Tg4fWsFPzSE

basically they ask to start yur system in nomodeset mode and then update the graphics driver and all .

Just check it