Cats don't recognise their name. How can I let them out?

[ShakespearesSisters]

So we have 2 gorgeous rescue cats (well, kittens) and they are 9 months old now. We had them at 3 months. I need to let them out as I want to have the windows and doors open in the summer.
They have been out on a harness.

My issue is that they don't recognise their names at all, or each others. (never happened with predecessors)
How do I get them back in if I can't call them?
Any tips?

Oh, photo payment included.

I don't call our cat in, he just comes when he's good & ready. He is a creature of habit though so I know when to expect him. A previous cat used to come if I rattled the door keys.

I rattle his food tin and he comes running! I said to the vet that I was worried he didn't know his name and they said he is choosing to ignore me! Cats live on their own terms!