The Ecobee products I have seen are made for the North American market and designed to operate with their type of furnaces and air conditioning using primarily blown air.
I only have very limited knowledge of these systems, but they usually have standard wiring letters to identify functionality, so for them, it is a simple matter for example, of connecting the furnace terminal marked 'W' to the Ecobee terminal marked 'W' with a 'White' wire, and 'Y' to 'Y' with a 'Yellow' wire and so on. Unfortunately unless you happen to know that 'W' is the signal for 'heating, 'Y' is for cooling, and 'G' is for the blower, etc., it won't help you much.
AFAIK, Ecobee products need 24 VAC to power them, and don't have either 'voltage free' contacts, or provide '230VAC' control suitable for the boilers we have here in the UK. I see you are in Croatia, but I would imagine that your boiler wouldn't be too different to the version we get here. It might be possible to install a 24v power supply for the Ecobee, and install a switching relay for the boiler control, but in terms of practicality I think it would probably be better for you to fit something else. Unless of course they now make an alternative version of the Ecobee for European markets.