Scammed by o2 operator

I signed an o2 contract last month, and in the shop I asked if I can pay the connection fee in cash. The operator said that I can. I paid 40 euros in cash (which I also never got the change for it)  and the operator said that he will deposit it into my o2 account. One month passed and now that I received my bill, I see that the connection fee is there. I reached out to o2 via chat support and it looks like that operator never deposited the money into my account.
It didn’t occur to me that the operator might just decide not to top up my account, that’s why I didn’t ask for any kind of receipt or something. I mean the whole idea of going to an official o2 store is that you can trust the operators there.
Now that the shops are closed duo to the virus outbreak, I can’t even go there and confront him about it.

Given the chance of me getting back my money is next to none, I wanted to make sure this doesn’t happen to anyone else.