Temperature does not indicate the power of the iron to heat a large mass of metal. My 15 Watt soldering pencil heats to 750 Deg F, but wimps out on anything larger than 16 Gage wire.
Check the power rating, should be 40W or 60W for what we do. I have an old school large iron for the buss bar, it is 200W. Whether you use 120V or 240V, Celsius or Fahrenheit, the wattage has to be adequate for the job.
Once you have sufficient power, you can adjust the temperature to work with the solder you have selected. Always use the lowest temperature that will flow the solder, typically between 600 and 700 degrees F will work.