Some of the comments here about ISP behaviour are crazy. Australia has had our fair share of fucking up the national internet infrastructure but at least I can pick pretty much any ISP and use any router I like. Haven't used an ISP supplied router in something like 15 years.
All over the US I have always been able to use my own cable modem and router. OPs situation is unusual, I am guessing its some bundle they have for a discount but if they were paying standard (ie ripoff) rates they could use their own equipment.
Used to be you could find out there what works and what doesn't down to the chipset variations. My experience was same as yours, as long as I matched provider capabilities, it worked.