Aha, both solder jumpers next to the esp8266 need to be joined, I just noticed on your pcb picture.
This picture here shows it most clearly: https://raw.githubusercontent.com/openenergymonitor/HeatpumpMonitor/master/images/buildguide/11.jpg

ESP CH_PD and GPIO15 Solder jumpers Both of these solder jumpers need to be connected with a small blob of solder for the ESP module to work.