Siemens KG33NX43GB/11 (Auto-Defrost) not working

Hey folks,

My trusty Siemens KG33NX43GB/11 fridge freezer has started freezing up and when it does the fridge stops working. I have tested a few things and what it looks like its the defrost system whereas the defrost heater does not seem to be doing its job.
I noticed a few weeks ago that the freezer door had not been shut properly how long it had been like that I don’t know but the food was half defrosted after this the problems seemed to start.

Problem – Noticed the fridge had stopped chilling and in the freezer the ice was building up heavily on the evaporator coils which was stopping the fan and as a result no cold air into the fridge.
I have a cousin is in Canada who is a heating engineer when I called him for advice, he said that it’s the onboard timer which has failed/failing and that’s why the ice keeps building up and fridge malfunctioning/heating-up.

I can’t find the on-board control system / PCB, if so, I might be able to test it, the defrost mechanism is built into the board which means that it will likely need a new board.

Is there anyone who knows these freezers and have an idea of what I should do?

Many thanks