I am in a connected community, we have a single SSID throughout the grounds including in our living space. Each person has their own pwd.

We cannot turn off the modem in the living space. So assuming SSID is "A", you are always connected to "A", whether in the lobby or your unit.

My unit has poor coverage from the modem, so I am getting a deco mesh units to place in the unit. Is there any downside to naming the SSID of the mesh network "A"?

Here is the issue. I go down to the lobby, my phone connects to A. I come back to the unit, it is connected to A. If I go to the balcony, where the signal from A from the modem is very weak but still connects, if the mesh is closer, will it pick up the A? If I name the mesh network B, when I come back to the unit from the lobby, my phone will still be connecting to A on a weak signal and not connecting to B.

Short of placing the modem in a faraday cage, I am not sure how to solve this.