I don't think you are looking for a full correction here, rather just for the part dealing with your situation. I would tell them:

"I chose your hostel because you offered free wi-fi. But I was not abe to use it, it has problems (describe the problem - I was not able to connect, or something). I had to go to (Costa Coffee?) to go online and that is not as convenient and it's not free because I had to buy a cup of tea every time. I expect that since I didn't get the full service I should not be paying full price. I think you owe me money because you advertise services you are not able to provide, so I should get a 20% discount (if you expect a 10% discount)."

Angry...

I am living staying in a hostel.
I paid money the hostel fee Aand Tthe hostel said you can use wifi in a the common room.
But I have been can't used the wifi by in the hostel due to it has problems.
If I still don't cannot use the wifi on Monday night.
I will try to say : " I paid the hostel fee which you said includinged the wifi.But I could not use the wifi in the common room.  I can get some discount" Can I get some discount"

is it make sentence?

I am typing in costa.

I know how it feels when the wifi cannot used.

Get some discount..

angry...

I am living in a hostel.
I paid money.
And The hostel said you can use wifi in the commonroom.
But I can't use the wifi in the hostel, it has problems.
If I don't use wifi on Monday night.
I will try to say "I paid money that included wifi. But I could not use wifi. Can I get some discount?"
 

I am typing in costa.

Sounds like a very frustrating situation!|

Good luck!