Airtel — Incorrect late payment charges

I have airtel mobile and broadband. In July they made a mistake of sending wrong link in broadband bill. As a result I ended up paying excess to my mobile ac instead of broadband. Airtel fooled me by saying that payment is stuck at the gateway.

After all the investigation when I realized that payment got credited to mobile instead of broadband, I reluctantly paid broadband amount.
By then due date had crossed. This goof-up happened because of tech mistake on part of airtel.

This bill they have charged me late payment fees. I am disappointed.
Contacting customer experience team and CEO is not helping. No one is interested to resolve this issue.
They are clearly cheaters.