VXOC Astra-J/GTC

So, my Astra A16 LET 1.6 Turbo recently sprung a leak as the thermostat housing had split.
Replaced the thermostat and housing and the 2 temperature sensors (one in the housing and 1 on the radiator), which fixed the leak. However, the fan is still constantly on maximum all the time, coolant level low message appearing (I think the radiator is empty as the reservoir is filled up and doesn’t drop), and the coolant temp won’t get over 75C using an OBD reader. 

No codes are present with the OBD reader and the heater vents are producing hot air.

Also, Since fitting the thermostat, the fuel gauge is stuck on empty even though it has half a tank in. This was working prior to having the Thermostat and sensors replaced. I assume this is related? It was working prior to this with no issues. 

Any help would be greatly appreciated.

You could try battery off for 30 to 45 mins then reconnect see if the brain has got itself stuck.

How much coolant did it take to fill system.you could then check this against capacity for coolant system see if you're short. Could be an airlock could be the radiator not full or air locked. Is it the correct thermostat?

I would start with troubleshooting them basics. No reason as far as I know that any of that would affect fuel gauge.