If you planned on using the Wifi and it wasn't operational you should feel ok writing in. You chose their product because of such features and that product did not meet all the expectations of service. If you get your $20 be happy. If they happen to only offer $10 I'd still take it and go on with your day.
I generally find B6 to be decent about their compensation handling when there are irregularities. If they do not offer anything just politely push back reinforcing that you had chosen them because of the product.