How do you get into the jail door in the cockroach dungeon on runescape?

You can't, there is a secret entrance round the back of the jailhouse however.

There is a second part to the underground jail... When you go in through the tunnel follow the path until you get to the larger cockroaches then head north until u see stairs, go up the stairs and in the room with the full size roaches will be a lever, pull the lever then go back down stairs and go through the jail door.

hope this is what you are looking for.

XweaselX