Pretty certain all free charging at supermarkets has ceased. Certainly Tesco/podpoint stopped last Nov22 and fairly certain all the other Lidl/Aldi/Waitrose chargers are not free anymore.
If you are using Instavolt then you will be DC charging at a 50 kW charger which has its own CCS cable which you plug into the Audi, whereas nearly all the supermarkets will have AC 7 kW chargers and you need to use the provided cable (type2) to connect to the charger.
This means it’s possible that your type2 cable is faulty, so it would be worth going to say a Tesco podpoint and trying it there.
You may get the first 15mins free, as the system needs you to have the podpoint app and ‘confirm’ the charge to continue past 15mins. It is payg 28p/kWh
If it fails then the cable is probably faulty. If someone else is charging also, you might ask if they would allow you to test with their cable to confirm.