To try to give you a different answer than the other comments here, let's try working backward from the goal. You want money. Who has money to give in this scenario? It's not the people fixing their own modems; if they did, they'd pay someone else to fix their modem, or they'd buy a new modem, or what-have-you. You probably won't see much in donations from them, if you go that route. No, the people with really ridiculous amounts of money in this scenario are AT&T.
So, next step: how can you cause AT&T to give you money? Well, what do you have that they want? They certainly might be depending on your documentation internally. They might not even know how much they make use of it (which means they have to assume the worst.) So "continued access to your documentation" can be something you can provide them in exchange for money.
Conclusion: revoke access to your documentation from AT&T's IP block (or just be lazy and take down the site entirely), and send them a legal notice requesting that they purchase a license (one-time or recurring, whatever suits you) for continued use of the documentation. Hold your documentation hostage, in other words.
Large companies deal with IP licensing all the time; for them, this is just one more thing to expense. For you, though, this will probably outweigh all the donations the page would ever make you. And you'll be making it from the people who made the crap modem, instead of the people who unknowingly purchased a crap modem. :)