Data dissapering and NO service

Hi,i have a 5gig data contract. For more than 2 years each month i ended with about 3gig left. The last couple of months my data does not last? I get a sms saying i have 450mb left and a minute after it that i have 150mb left? The line os not even that fast. I phoned 114 asking where is my data dissapearing to, the answer i got is that the number in question is a data contract that only use data and she can see data is being used,no calls made. Well its a router and it cannot dail. I have a sim only contract,bought my own router due to out of stocks when i upgraded, do i get charge if i cancel a sim only contract? Can 1 person phone me that understands me,that is not eager to transfer me? Customer care website states we need to phone them due to email not working "data issues also?". Please help, i use this for work purpose. Its urgent.