Activating apartment building buzzer "Door" open button remotely
What you want to get is a one channel, wireless remote control, like this one from Amazon:



The receiver has a relay contact output, that can be set to either operate momentarily as the remote button is pushed (which is what you want), or operate in a latching mode, on, off, on, off (which you don't want).  This is selected by a small white mode button on the unit.  One of the reviews explains the settings (apparently the documentation is not so good.)

You will want to wire the relay contact in parallel with the Door button contact on your existing unit.

The unit runs on 12V, I don't know if that voltage is already available from your box or not.  Otherwise you will have to make provisions for it.