Anti freeze will freeze. If it was that cold, we used to place cardboard over the front of the radiator to prevent friggen COLD air from being pulled in to freeze the liquid in the radiator even more. Have your anti-freeze to water mixture checked. You may not have the correct mixture for that cold weather, or even old or the incorrect anti-freeze will become solid and prevent the coolant from flowing. It is not your thermostat, it is a blockage in the coolant system at that cold temp. Now -39 Celsius is a -102.2 degrees Fahrenheit. Where the F--- were you. Antifreeze, no matter what you have will freeze at that un-godly temperature. It will freeze the brass balls of a monkey. (An old Navy term. The old wooden battleships used brass cannon balls to shoot. The iron plate made with divots to hold the brass balls was called a monkey. When it got cold, the iron and brass would not expand at the same rate, so the brass balls would fall off the monkey.)