Airtel — airtel using my connection

I am using airtel broadband. On 3 occasions someone without my knowledge has uploaded over 100 gb of data from my airtel broadband account. Chances that i was hacked are very remote and no devices were connected to my router for so long that over 100 gb would get uploaded.

I now believe that someone in airtel broadband has used my connection. Their security is faulty. There's no way to change or set password. It is given by airtel so their employees know it. There's no mac authentication so someone can log in from elsewhere.

Please be careful while availing airtel broadband services