If I remember correctly (you'd better googel it to be sure), PWM case fans use a steady 12V supply, plus a pwm signal.

I'd buy an ESP, connect a temperature sensor, put the Tasmota firmware on it and be fine. The programming is a one liner then (in that weird tasmota rules language).

If it turns out that the fan needs a pwm "chopped" 12V supply instead, then you need to add a MOSFET and 2 resistors to create that.

P.S. that module from the other comment is better.

https://acinfinity.com/closet-room-fan-systems/

They have a lot of products, including other fans and thermostats. I've had their media cabinet fans running 24/7 for 4-5 years now with no problems. Highly recommend it.

Otherwise you could use something like this and a standard 12v power adapter https://www.tindie.com/products/mmm999/dc-12v-four-wire-thermostat-pwm-pc-cpu-fan/

If you don't mind playing around with a Raspberry Pi and a few jumper cable, have a look at my repo. Half a year ago, I built a fan controller to regulate temperature and air flow in my server cabinet with 5V PWM fans. It is not finished and rough but it works. After moving in a few months, I will have the time to work more on it.

If you have questions, feel free to DM me :)

I bought this one, but I and then an extra standalone fan, to have neutral pressure in my network closet, and it has never gone above 30 Celsius.

AC Infinity AIRPLATE T7, Quiet Cooling Fan System 12" with Thermostat Control, for Home Theater AV Cabinets https://a.co/d/0jgj41Ie

There are case fans with a temperature sensor. They already control the speed themselves depending on the measurement. The only thing they need is 12V.

For example the Arctic F12 TC.

If you mean running the fans in 240vAC, Comair Rotron make fantastic fans for this voltage. If you mean a regulator circuit and any old 12vDC fan, sorry for misunderstanding.