If you've done the plane delivery (mission "Delivering the truth"), then you don't have to wear the Epsilion robes anymore... not for now.
You've done all you need to do. It pop-up for me after a few in-game day. Just continue the main storyline or collect collectibles and the Epsilion contact point will eventually appear in the desert, west of Sandy Shore Airfield.
Before going there, be sure to put back your robes... and have 50k$ for next mission.
Good luck with that "pain in the a**" mission... you're gonna need it. Running 5 miles in desert... pffffff, what a time-wasting mission.