Gear S bought in US not working on GSM Europe

I bought a Samsung Gear S in the US (Full unlocked) Model Number: SM-R750A, Software version: R750AUCU1AOB1. It works OK on everything except for GSM. I put my carrier SIM card on it but it has no connection. I checked and found that my Gear S works on GSM 850/1900. I think that this should be changed to GSM 900/1800 to work in my country (Europe).
Anyone any idea??? :confused: