Dell XPS 15 9550-1370 HDMI stream issue

Hi I tried to use my XPS 15 for hdmi to coax streaming via HDMI output with HDMI QAM Modulator. I have an issue - I can't get 1080p@60 , it works only under 1080p 30fps(GTX 960M should be powerfull enough for 60fps). What could be the problem? I use H.264 encoding and QAM DVB-C modulation. Thank you!