Can't you buy your own cable modem and use that?
Unfortunately no, Armstrong did not support bringing your own modem. It could technically be done prior to DOCSIS 2.0 but it was never officially allowed.
In the US providers are not required to allow users to own their modem, but they have to provide a modem for free if you can't bring your own.
This is mostly fine as long as they offer a plain modem, it's when they don't offer a model without its own (almost always absolute garbage) firewall/router built in that it's a problem. IMO it shouldn't be allowed to call something "internet service" unless I have a handoff I can plug in to a firewall of my choice and have a publicly routable internet address directly on my box's WAN port
That is a problem, but you can work around all of that. All of the ones I've seen have allowed you to just turn that stuff off, but even if you can't, you can approximate it by setting router rules that just forward everything to your real router.
The BGW320 gateway that AT&T provides with its 2g/2g and 5g/5g fiber services has a NAT table limited to 8192 tracked connections. Let me repeat that, a 5 gigabit symmetrical XGPON service has a NAT table that makes a 2004-era garbage tier device look good by comparison. It does not support a bridge mode, only "IP Passthrough" or the standard default NAT "DMZ" mode commonly seen on consumer hardware. Both of those rely on the tiny NAT table, so anyone who wants to establish more than 8192 connections at one time has to get a routed block of static IPs that allows the NAT to be entirely bypassed.
At least officially, for now there's also a method to clone your ONT identifiers on to one that works as a plain bridge, which I have the parts for but haven't installed it yet.
Their high speed DSL "U-Verse" services have a similar issue with a bad mandatory device, those require a complicated bypass involving passing through 802.1x packets to the provided router so it can authenticate while you then use your own to actually send/receive internet traffic.
Personal experience with Comcast in the bay area. I now have my own modem.
The cable company resisted it but ultimately published a how-to as well.
https://www.ziggo.nl/klantenservice/apparaten/wifi-modems/ei...
But for the record:
If you just sign up for Comcast (XFinity, whatever) and use the internet, you are renting a modem/router combo from them for about $10 a month, in perpetuity. You can buy an off the shelf modem/router for a couple hundred bucks, and use that instead. It'll pay off in a year or two. You have to call Comcast up and activate the device, but it's otherwise a completely painless process.
Further, my experience with Comcast specifically is that every three months, they "forget" that I don't rent a modem from them and just start charging me the rental fee anyway. Then I have to call them up to tell them to remove the charges and stop adding new ones.
Not only can you do it in the US, but the cable company will give you clear instructions on how to get it set up and running on their system.
And it's a good idea. Not only to avoid paying the modem rental fee, but also because the modem provided by cable companies tends to be pretty awful.
Then they show up and use their shiny new Viavi (formerly JDSU) OneExpert... and see the same thing and fix their shit. ;)