@Stab power failure can be a reason, so this can be a good test.
If it fails again, then we are sure that it is not the 3.3V from the pro mini who is to weak.
If it fails not, we have the solution
If I understand correctly, the door bell button is connected directly to the pin 3 of the arduino. When pushed it takes the pin 3 to the ground GND?
How long are the wires between the button and the arduino? Are they shielded?
Long wires can act as antenna's and inject directly faults into the pins of the arduino causing crashes of the Arduino or even the dead of the Arduino.
The best way to avoid these problems is to use an optocoupler.
Search on Google on the keywords "button optocoupler arduino" and you will find a schema.
You can find a good explication on: https://www.electronics-tutorials.ws/io/input-interfacing-circuits.html
If an optocoupler is a bridge to far, you can also start with putting a little 100nF capacitor directly between Pin3 and the ground, as close to the arduino as possible. It will act as a high frequency killer.
I don't see strange things in your sketch, so it should be oké.
Must the node act as a repeater?
If not, set the define in comment to have the bare minimum....
On a pro mini, there is no standard serial connection, I think.
Otherwise you could directly connect a laptop to see the debugging messages.
