At both sides (where the 2 buttons are), i have 3 wires from the wall.
In the scheme you posted, I would need almost 4 wires
Surely is the neutral wire in lower switch. Instead of returning to lamp you must to take it to sonoff. You must look for where it goes and connect to S2 terminal of sonoff to this wire disconnect it previously of neutral network. And connect neutral network to neutral in of sonoff.
Or it can be connected of the other way, hot line in lower switch and neutral in upper switch. The idea is the same.