Hathway — Refused to pay my security deposit amount

I taken connection from hathway & they asked me to pay 400 as a security deposit. I paid. They said once you return the router box, we will refund that amount. I returned the router box and they given that router box to another subscriber. I am getting text messages like Renew your account. Many times, I called Customer care and they said we will take this request, but 6+ months gone.

Account no: [protected]