thermostat valve restricts the flow of coolant through the radiator when engine temperature is cold and once engine temperature reaches it optimum range it allows the coolant to flow through the radiator thus maintaining optimum temperature. therefore you have to remove it before flushing.
Flange is the housing in which thermostat fits
therefore get the thermostat removed. flush the radiator with tatri solution, then flush again using baking soda solution. then rinse multiple times using water, ensure radiator is good (not choked and not leaking), remove all water, re install thermostat, add ZIC diluted coolant