Hi guys!
If it's not getting hot enough, then, as you intimated, it could well be the thermostat even though it was changed.
Try this -
After a drive, or not, if you sit with it stationary, with the heater off will speed up the experiment, maybe a slightly fast idle speed, and see if it will then get hot, and hot enough to bring in the radiator cooling fans.
If it does, quite possibly the thermostat.
I had a Ford Galaxy 2007 1.8 diesel that would not get over around 65C, even with most of the rad blocked off. Never did figure it out.