We had the same headache in a 1940s place with plaster over metal lath and the monitor would drop the moment we closed the door. What helped was moving the parent unit to the doorway shelf instead of the nightstand, cracking the door an inch, and shifting the camera a couple feet to avoid a big mirror that was bouncing the signal. I also turned off Bluetooth on my phone when the monitor was nearby and stopped wireless charging right next to it. One more gotcha is voice activation or eco modes that let the screen sleep which can look like a disconnect, bumping the sensitivity up made it wake faster. Not pretty but it worked.
