my best guess upon your info is that the heater (air)control servo actuated flap is stuck in the closed position. This blocks the air trough the heater matrix and only allows ambient air (or chilled air) to enter the cabin.
The feed pipe to the heater matrix is always circulating and not plumbed in the thermostat circuit afaik
- in fact the heater matrix is always hot (even with the air flap closed).
with rear undertray off, you can locate the smaller hoses going in the right sill and returning from the left sill, and assess their temperature. Follow the hose from the electric heat soak pump mounted to the coolant reservoir; that is your heater matrix circuit.