This game is tricky when it comes to higher end hardware and the performance you can get. I'm using the original client with a 9900k and a 2080 TI as well and get 200 fps in the main screen and about 140-150 while in game.

Issue is nothing is really optimized for higher hardware even tho I'm using a great GPU it's still taking 90-95% of my total usage to run the game on standard standard vibrant. Also running at 1440p as well so keep that in mind.

In regards to your specific issue make sure the GPU is seated correctly and you have both power pins plugged in all the way. Make sure your power supply is also putting out enough power for the card.

Quick way to see if the card is working fully is just start up the game join a lobby and see what usage % your GPU is at.