I got them working.
Default Smartthings Fibaro Smoke Sensor device handler in IDE is not correct and should not be used with FGSD-002 EU sensors.
Found this link:
There was link to official Github:
Which includes “Fibaro Smoke Sensor” (updated 6months ago)
Went and checked device handlers code and it was quite easy to notice that it’s different than the default “Fibaro Smoke Sensor” in IDE.
Copied new code and created new device handler. Then one more time full reset all sensors by pressing sensors middle button 3 seconds, when yellow light -> pushed shortly again. Red flash and sensor is reset.
Then normal way added new device from mobile phone and pressed sensors middle button three times to achieve pairing mode.
Shortly sensor was found and it was recognized as “Fibaro Smoke Sensor ZW5”. Now this is different information than earlier when sensor was just found to be as "Fibaro Flood Sensor(?).
Gave device a suitable name and then saved it. Went to device information from mobile app and pressed sensors middle button one time. Sync started and soon temperature and battery level was shown.
It’s important to do that button press before changing settings. I you do not press button one time before configuring settings sync will never success.
So finally I went and configured sensor settings and pressed save. After that middle button has to be pressed once again so that new configuration is synced to sensor.
Made a ticket to ST and just sent them email if they should update default device handler for Fibaro Smoke Sensor. Thanks Simon for you help too!!