I had an Audi A3 Etron plug in for two years before my GTE Advance. The heating module that is used in EV mode had to be replaced twice. The first time it went wrong it took some time to realise there was no heating in EV mode as I was doing short journeys. You should start feeling warmth coming up after 1 to 1.5 miles. If not your heater module may not be working. The dealer didn‘t believe me until they plugged diagnostics in the first time.
I do find the heating a bit lacking around the legs, but I thought that was just me! The heating does work in both EV and ICE mode and heats the cabin generally, but does lack a bit of oomph in the leg area I think.