Maps are sometimes randomly generated in a way, where areas are inaccessible by normal means. But enemies can still spawn there. Usually this is unintentional and considered a bug but in this type of map it IS intentional and the briefing tells you this.
Without looking at your save, there's probably a room or several rooms without a door. You have to make your own openings. Dynamite isn't strong enough, use High Explosive.
With the most recent update, melee weapons have also become capable of destroying terrain, so you could try that, too. I haven't played in a while, though, so I don't know which ones would be best for the job.
