I have a door that has a chain lock, like this:
Sometimes, my roomate locks me out with this thing. How can I be able to open it from the outside?
Image taken from the above site.
The one I believe in mostly is taking a piece of string and tying it to the chain, then threading it over to the outside of the door and pulling chain open. But I can vouch that all of these methods work.
For the rubber band method you can use a string that is tied, it doesn't have to be a rubber band. And if you are small you can stand on a stool and stick your hand in the door that is cracked and open the door that way. Wikihow also had a method, but it includes pushing against the door.
For the type in this pic, with the door ajar, reach in with the key and unlock this chain that has it conveniently built-in for this very purpose.😁