I've attempted to replicate the following.
I've changed 8k2 to 10k and 2k7to 2k4 since I didn't have the correct resistors but should still be fine to bring down 15v to usable 3 for esp32. (I've skipped LDO)
I am just not quite sure that my breadboard looks correct, can someone spot something fishy?
What should this circuit do?
What is the voltage on pin 8 ( as the esp32 is connected to this pin via a 20K resistor )?
Pin 6 seems a one line rx/tx comunication if you are using 10K and 2.4K you should have 3V with 15V input voltage
What mosfets are you using ?
What does not work ( does tx work, rx work, the other signal which I interpretate as 'power request' work )?
Regarding the photos... I'm seraching the phone number for 'semiconduttors abuse' ( there should be one ; - )
Don't do those 'contortionism', mount mosfets ( with unbent legs ) in 3 adiacent rows and connect resistors, wires as necessary...
This supposed to be able to send and receive data through RJ45 between ESP32 and another microcontroller.
I'm using 2N2222.
Pin 8 is not connected to ESP, so only using pin 6 for TX/RX communication. However, I do not get any data to or from (the voltage here is measured at 2.36V not sure if it is too low).
Attempting the following: GitHub - Ordspilleren/esp32-linak-desk-control: Control your LINAK desk with an ESP32!
(guilty, I promise to make a significant contribution to the foundation of abused semiconductors)
yes 
Ok if you are using a 2n2222, which is a transistor, you have to add a resistor ( 2.2K ) in series with the tx pin ( which connect to the base of the transistor, connect Emmitter=Source, Base=Gate, Collector=Drain ).
Where is the 2.6V? On the rx pin is ok, on the 6 pins you should have the 15V ( as you are converting 15V to 3V )!?
What is the nPWRreq used for ( is it necessary to pull it down to turn on 'the other side of the circuit )?
If you have an oscilloscope or something similar, do you see on pin 6 the 15V modulated by the transmission ( similar on rx pin but 3V modulated )?
This is not a mosfet. It is a bipolar transistor. But your circuit is expecting a mosfet and therefore it can never work.
Ah, I see, then the wiring is completely wrong with this transistor.
I do get around 13.6V on pin 6, my assumption of 15V is not correct, though down to 2.6V should still be fine.
nPWRreq is used to "wake" the device but it can be done manually so to avoid extra complexity; according to description (nPWRreq is left unconnected (may be pulled down to GND after VCC voltage appears to save some power).
Unfortunately I do not have oscilloscope but I do have some IRFZ44N, so I will try those instead of 2n2222.
You should not see any voltage on pin 6 unless the RJ45 is connected to something
That is correct.
Here is my new circuit with ESP and IRFZ44N
RX(yellow) to pin 17, TX (green) to pin 16
Does it work?
Well something does interfere with an external controller box having this circuit plugged in but it does not look like ESP is getting anything, in fact, voltage is near zero at pin 6 now 
don't use irfz44, the 2n2222 is ok, just add the 2.2k resistor ( or similar ) in series with the base
As said connect Emmitter=Source, Base=Gate, Collector=Drain ( = means 'in place of' )
the 2.6V on rx is a little bit on the 'low side' ( esp32 datasheet says ViH min = vcc*0.75 = 2.47V ), but should be good
Are you sending data or receiving data ( or both )?, can yoy preent on the serial debug any chars received ( in hex format for example to be sure to print also non printables )
The circuit is ok, but you are sure that the lin packet is ok ( or you can sniff a lin communication )?
Must admit know nothing of lin
OK,I will swap them back to 2n2222 leaving everything as is now, only adding an additional 2.2k between base and TX pin. It's both sending and receiving but debugger is quiet.
Hopefully will get some data in debug after implementing the suggested.
Ok also check 2n2222 pins: Basics of 2N2222 | NPN Bipolar Junction Transistor Guide | DesignSpark
