If we ever want to try it again.
I grabbed this description from Reddit, which seems to suggest it can be a bit of a waiting game:
If you're running solo, you blow up the bikes then run to the apartment door. You might have to sit on the marker for quite a long time, but eventually it will trigger.
A part of it seems random, because sometimes if you can't get in through the door, you can try the garage and get in no problem. Most of the time however it didn't work for me.
The only absolute 100% way of getting in straight away that I found is killing yourself after the bikes, then running for the garage marker as soon as you respawn and entering before the bikes are blown up, so you basically have to be faster than your explosive-happy teammates.
But honestly I can count the number of times I had to resort to suicide on one hand. 99% of the time if you camp on the apartment door marker long enough, you'll get in.