Operating temperature

If half way is where the gauge stays then thats probably normal operating temp. Is cold air blowing through the vent or nothing ? Do you hear anything open or close under the dash when you move the selector for heat ?

You are thinking that I said that the gauge is going to it's normal temperature but it is only halfway to where it should be. And yes the dampers open and close but the air coming through doesn't get warm.

are the coolant fans running all the time ? , is there the right thermostat in the motor ?,

if you can reach the heater hoses that run through the firewall to the heater core see if they are both warm/hot

Replace the thermostat with a correct Subaru brand t-stat. Burp the system when you are done.

Make sure you fill in the coolant and check for any coolant leaks. Once you fill in the coolant heat should come through normally if still there is a problem you will have to bleed the air from cooling system