After receiving some help with the code and a new version of Artnet from Yves BAZIN (His work can be found at https://www.youtube.com/watch?v=4viy1FxyLiY&feature=em-comments and https://hackaday.io/project/158268-5904-leds-panel) I finally managed to have it working. For 600 LEDS I have used 4 DMX universes with 150 LEDS on each DMX universe. No more delays on 3^rd and 4^th universes. I will post the code later on. I have inserted a picture of the prototype panel in EVA.