Thanks all for your suggestions.
Looks like the site operators are finally moving on sorting out a proper intercom unit.
In the interim I ended up making a doorbell system with D1 Minis and ESPNOW. They're keeping the gate open but are locking the front door instead.
Essentially a battery powered front door button unit sends a message to an intermediate, and that then sends on to the bell unit in the lab. If the signal wasn't strong enough I could add additional intermediates to get the message from the door to the lab, all sorted!