My previous question was closed as duplicate but it is not, this question asks about how did Neil get through the shut tunnel, which is different to how did Neil unlock the gate. Please don't close.
At the first part of the 10min attack, Neil was trying to warn the protagonist and Ives driving the Hummer behind them, after they went in, they trip the wire which detonated the bomb that shut the tunnel's entrance.
At the end of the film, Neil again goes back to unlock the gate, presumably through the same tunnel entrance, because we saw him walking backwards to the the tunnel in a another scene.
The cave has only two entrances, the tunnel that got shut by the bomb and the top oopening that the Protagonist and Ives got pulled out from, you can see the map of the cave in Ives’ briefing before the 10min attack.
Also you could see how bad the entrance is blocked in the movie, lots of rocks and slabs stacked.
How did Neil manage to get through the tunnel entrance second time where he decided not to even try the first time?
You can also see the issue for reversed Neil, the protagonist and Ives go in the tunnel, bomb exploded, tunnel entrance shut, Neil is inside dead, he wakes up and ran backwards to a shut entrance.