I've tried slot 2 with the 1080 card but only tried slot 1 with the 1070 one but no combination worked so far.
I found the issue! I had a hunch that it might be something about the video card firmware and after some googling I found an nvidia page about older cards not being compatible with DP1.3 and DP1.4 and that 'Without the update, systems that are connected to a DisplayPort 1.3 / 1.4 monitor could experience blank screens on boot until the OS loads, or could experience a hang on boot.' which is exactly what my issue was! After I updated the f/w I can get to the BIOS as expected! I just wanted to share this for the archives in case anyone will have the same issue in the future. See the nvidia link below.