It seems there is a common glitch with trying to complete the Roosevelt Island Stronghold main story mission on the default/normal difficulty.
The glitch occurs during the "Secure the Entrance" step of the mission. There are ropes that need to be climbed in order to get over to the next area of the mission, however the ropes are not released in order for players to grab hold and escalate themselves upwards.
I have posted a video of my live game-play demonstrating this glitch. No need to watch the whole video, only first few minutes. The rest is me trying to re-spawn outside of mission and try again to see if this changed anything (which it did not).
I will attempt to go back to a previous save or start mission over (not sure this is an option?) and see if that does anything, or try to change the difficulty and report back if that changed anything.
Thanks for your help.