Two possibilities. 1. Coolant Temperature Sensor 2. Thermostat CTS is under a tenner and very easy to remove and replace, just need more coolant and ninja hands to minimize coolant loss. If it still doesn't work then Thermostat is the next point, if still not then water pump.
Plastic, plastic welded in. A gap that large will need to be properly cut out and replaced. Filling it up with anything is a cheap skate way of doing it that will only fail sooner rather than later. Especially in this weather.