"Escape through the portal" is a mission objective in Server Bridge Paris 1944
Head over to the left and around the corner. Climb up the wall using the crates. On this landing just head for the fencing. Start going up using that. When you get to the top, shift over to the middle pillar. Use that to climb upwards and reach the middle section. Use this section to finish the climb and get Arno through the portal. The open portal above him will close. They will open a second one below you. Jump from the top and down into the portal.