Every month I received a message on my phone that says I have reached my maximum limit and I could have some limitations or experience slower service until I renew my bill is this false advertising? Or is it legal because they put it in their disclosure and find print but not in the advertising.
At common law, what a contract says is what a contract does and no part has any more or less weight than any other part. However, each jurisdiction has consumer protection law that restricts what contracts can do.
For example, the Australian Consumer Law makes it illegal for businesses to engage in deceptive and misleading conduct: if using "unlimited" in large print and all over their advertising would lead reasonable people to believe it means unlimited and unthrottled then having small print explain that it doesn't may not be enough.
It also lowers the bar for equity in standard form contracts from unconscionable to unfair if you want to challenge the legality of a term.
Or is it legal because they put it in their disclosure and fine print...
It's all in the contract you entered into when you clicked/signed/agreed to the phone contract, and you agreed to the way they define "Unlimited."
Your data may be unlimited in total, but the speed at which you get your unlimited data may be limited. That's because they save money on bandwidth and they may also be "encouraging" you to upgrade to a faster plan.
If they clearly break their contract in terms of what they define as "unlimited," then you have a case.
But I doubt they are going to clearly break your contract and millions of other contracts. It can happen, but it would simply cost the company too much money. See such Google results as https://www.prepaidphonerefund.com/ and https://arstechnica.com/tech-policy/2016/03/att-defeats-class-action-in-unlimited-data-throttling-case/
"you got it buddy the large print giveth and the small print taketh away..." from http://www.tomwaits.com/songs/song/322/Step_Right_Up/