Many engines need to have an air pocket bled out before the thermostat will open. Follow that upper radiator hose to the housing it attaches to on the engine and look for a hollow bleeder screw. If there is none, look for a plug or sensor nearby that can be removed. Fill the coolant until it comes out of whatever you opened. If you have to unplug a sensor to remove it, do that with the ignition switch off to avoid setting a diagnostic fault code and turning on the Check Engine light.

There should be a bleeder on the thermostat housing if it still has a genuine housing fitted.