To unlock fast travel to safe houses across Glass, you need to complete the various Gridnode side-missions, the first of which is required to be completed as part of the story after the Savant Extraordinaire mission. Subsequent Gridnodes for other districts will unlock as you continue playing through the story.
Each Gridnode is an intricate platforming puzzle inside a server stack, where you have to find your way up a circuit-board laden complex unlock a CCU. Lasers block your path, and while you can safely pass through them, only by completing an entire run through without detection will you unlock the 'Undetected Surge' achievement.
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Mirror's Edge: Catalyst - Gridnode Run
How to complete the mandatory side-mission and unlock fast travel to safe houses within Downtown and Anchor.
Mirror's Edge: Catalyst - Gridnode Downtown, Gridnode Rezoning, Gridnode The View
Unlock fast travel to the remaining safe houses in Downtown, Rezoning and The View.
When done, go back to: Mirror's Edge: Catalyst - Guide
Unlocks: Fast travel to The Lair (Zephyr Transit Hub, Downtown), Plastic's Lair (Crystal Valley, Anchor), Eden Village (Eden Village, Anchor); 'Benefactor' main mission; The Metagrid and Gridnode Downtown side missions; 'Devastation' achievement
Reminder: This side-mission is encountered as part of the main story, after the Savant Extraordinaire mission.
Climb the circuit board in front of you to enter the main building proper. Here, you have to make your way round the outside of the room while avoiding the blue lasers. Wall run over the first double lasers on the left, underneath the one above and across to the end wall. With the walk way ahead, make sure you wall run high enough to clear the laser just underneath, then climb the circuit board ladder and round the outside.
At the far corner, this is much easier if you cleanly wall run and leap onto the walkway, but if you fall short, you can crawl underneath the laser, and climb back up to the central walkway at the far side.
Once you're in the central walkway, go to the other side of the floor, climb the circuit board ladder, turn around, and across on the overhead pole. Scale the two ledges and you'll notice three lasers blocking your path through the middle; climb to the corner and wall run over them. Go to the other side of the wall, up the circuit board ladder on the right, turn around and to the tinted room in the corner.
From here it's plain sailing; head up the circuit board ladders, through the vent, dash through the conference room and down the stairs and use the computer. If you managed to do the mission without getting spotted, you unlock the 'Undetected Surge' achievement - but if not, there's plenty of other Gridnode missions that allow you to unlock it - see below.
Head back up the stairs, down the elevator to return outside. Run ahead to trigger the cutscene. Now you have to evade the K-Sec alert phase. Using Runner's Vision and the map as your guide, run past every enemy, as with enough momentum their gunfire won't affect you. Don't worry about the radar and overhead choppers, as when you get back to Crystal Valley safe house the alerts will die down. Chat to Plastic in her lair to complete the mission.
Note that at this point you can now infiltrate future Gridnodes to access other safe houses across the city of Glass. Right now, three have already been unlocked - Plastic's Lair in Crystal Valley, and The Lair back in the Zephyr Transit Hub, and Eden Village in Eden Village. Fast travel to the latter so you can get started on the next mission, Benefactor.