Hi @armit
I understand your last point. Not everyone knows everything about how the mobile network works and if you receive advice in a shop, you can assume that it is correct. I wasn't there for the interview, so I can't comment on that.
With the S21 you should be able to use 5G, because it is made for that. The S20 you may have a 4G version, not 5G. So I don't dare to say an your S20, you will have to look at that yourself. But the S21 should just do it. And whether the router works, I really don't know, I can't give you any information about that.
I would go back to the store and file my complaint and ask what they can do. I can't really give you any advice on what you have to do. There are Vodafone employees online on the community but I don't think they can do much for you either. You actually have a dispute with the store and then I would resolve it there.
I totally understand you. If you are not informed and you get to read and hear a nice story, then you take it. Sorry I can't do much for you, I don't work at Vodafone. You really have to work it out with them. If you are still within the 14-day cooling-off period, you can always cancel your subscription.
Sorry for bad English, I have use Google Translate 😉