Siemens KG39NA90GB alarm and ice build up freezer floor

The alarm sounds preiodicallly and is a pain beacuae it keeps coming back after the minimum period (2 mins?) then disappears for somethimes weeks. The alarm can be temporarily cancelled by pressing door button or sometimes just pressing door closed. Eventually I clear it until next time. I clear ice from freezer once a monthish.
I’m hoping this is because drain is blocked and not pcb or auto-defrost problem. I have checked the back and there seems no issue with the u-bend of rubber pipe to the collection well. There was no water in the well but it could i suppose all have been evaporated off.
I can see two screws in the freezer back internal panel. The ice is coming from the upper right hand side as you look in and running down and pooling on the internal floor.
What am I actually looking to clear in this model. Is it a drainage hole or a pipe or some sort of funnel effect thing behind the panel?
I can’t see a drainage hole in the fridge compartment and I’m assuming it is icing not food/gunk but nor sure.
Any help appreciated. Keith