In khyber the thermostat has no rubber seal around it rather needs a gasket easily available for it. The flange(edge) of the valve seats in a groove to make a leveled surface for a gasket.
At operating temperature, the valve should open, and you should be able o see the whirling in the radiator with cap removed. Take care not to open the cap when hot. The 90% temp at indicator means that your engine core temperature is high NOT the radiator. Check the valve operation first.