Do not understand corporate clients

I bought a modem for my laptop. I ran out if data and can not buy more. I opened the accounts and is the only person with authority to deal with the account. Now they say I am not the account holder and need to send a letter on a cmpy letterhead. I did all this when I added the modems to the account. Why did they not link my details the same time? Thank goodness my contracts finish in April next year.