This is a dangerous game you're playing (despite the Robinhood like dynamic -- trust me I hate ISPs too, though I will say AT&T fiber has been quite reasonable). You can be sued for all the lost payments plus interest, and likely will be when/if they find out.

It's the same thing as getting an unexpected raise on your paystub, if it's in error and a reasonable person would believe it's in error, they're within their legal right to take the money back.