Trying to flash ME81H with Tasmota

[Kolia56]

I compiled Tasmota 12.4.0 including specific define statements as described here. I even set various WIFI options in the code in order to get access to the unit as soon it is flashed. I used VSC with platformIO extension. Everything when smoothly.
After that, I made numerous attempts to flash the binary directly on board. It failed whatever options I took. I could not event use esptool -flash_id. So I decided to order ESP12S chips. I had no issue flashing one unit. Template has been set and I checked that commands sent to the unit worked as expected. I also checked that MQTT worked accordingly. So I unsoldered the chip on ME81H board without any specific issues (I use a professional heating station), cleaned the board and soldered ESP12S chip using a binocular. Thermostat has been reassembled and powered on. I was please to see that communication was working. But I soon discovered that ESP12S does not communicate with the hardware platform. In other words, current temperature displayed on the panel is not reported, when TempTargetSet1 xx is used, it is not updated on the panel. Same with changing °C to °F for instance. Command THERMOSTATMODESET 1 has been issued initially in order to activate the thermostat driver.

So at this point, having tried many things I'm out of ideas. Does someone have an clue of what could be wrong? Did I miss something?
Thank you so much in advance