Decorating: how can I hide protrusion in wall?

I hope someone would be kind enough to help me.

We recently had a bathroom cabinet fitted. As it required wiring to the light switch an electrician used a tool to cut holes into the wall so he could feed the cable through and connect it.

He then replaced the holes somehow (they seemingly went in and out in one piece, I have no idea how they were stuck back in) and said that all I would need to do is sand the edges and paint over them.

My problem is that I have tried sanding them (with a fairly fine sandpaper), but there is still a slight circular ridge that is still bound to be prominent and unlikely to be hidden by even a couple of coats of paint. *Please see attached photo*.

I doubt I have the skill to skim the wall myself, and currently really can't afford to get someone in to do it, however I've seen a few options online that I think might be worth giving a go myself, including using a base coat, or a product such as "Everbuild Filler Coat".

I'm hoping that someone might have experience of a similar issue and be able to advise me please.

Best regards