It’s very situationally dependent. In many cases though, the only thing restricting you is the ISP not giving you the information to do it yourself, and sometimes, just sometimes, some kind of code or MAC address that limits what devices can operate on their network; though that’s usually set up for billing.
If you use any kind of PPPoE, the MAC address problem usually isn’t a thing; this is normally DSL/Fiber… not all fiber, just some. PPPoE needs authentication which usually means username and password login to do AAA for the client, because of this, there’s little to no security on the last mile. All these technologies are based on standards. Fiber is usually GPON, DSL has several standards, but modern DSL is usually vDSL or vDSL2, or some variant thereof, and Cable is generally DOCSIS 3 or 3.1. There are exceptions, but they’re not common.
The key is to find which specific technology the ISP is using, and find alternatives. In the case of DSL, it’s generally finding a DSL modem that uses the same profiles and annex as the provided modem; beyond that, plug it in and authenticate with PPPoE. Many PPPoE type providers use a circuit number and/or VLAN, so that generally needs to be set along-side the PPPoE credentials.
For DOCSIS, it’s a bit sticky, since I know of many cable providers who authenticate endpoints based on the MAC address of the modem; in which case, you not only need to find a modem that can support the protocols in use, and the channel widths (eg. DOCSIS 3.1 16x8), but also one that you can modify the ISP-facing MAC address on the DOCSIS interface to match the one they gave you.
For Fiber, things can be sticky, but often aren’t. The ISP can, but often doesn’t filter on all of the following: MAC, SN, SLID. All of these values are sent to the OLT (ISP side of the fiber), and it could fail on any one of them. For me, I’ve had success with the G-010S-A SFP module, and if you look around the internet, you can find a git repo that actually has all the commands to modify any/all of these values to match them to whatever the ISP provided to you. The most difficult is getting the SLID, since it’s not published on the outside of the modem. I managed to get my local GPON’s SLID from a G-010S-A module that I hijacked from a working modem; in that case it was a string of all zeros.
The information is out there if you look hard enough, and with a little bit of cleverness and ingenuity, you can usually find anything that’s missing.
I work in Networking (aka network engineering, aka a bunch of other titles), so this all comes very naturally to me; to give you some examples, one DSL modem delete I did for myself was to pick up an EHWIC-VA-DSL-M for a Cisco ISR router, after some configuration magic, which I won’t get into here, I was able to get it to connect to my ISPs DSL line, after a bit more configuration magic, the Cisco was handling all of the traffic from my network to the DSL. It was a very clean setup, only requiring a single phone line from the wall plugged into a module on the router, then on the other side of it (over ethernet) was my network. That’s a fairly advanced one, but I’m pretty proud of it. Another case was a friend on the same last-mile provider in my country, on a fiber line, where I removed the garbage modem they gave him and replaced it with a G-010S-A GPON to SFP module, and plugged that more or less directly into the router he owned. In each case, I shaved off a few ms of latency, and bandwidth was largely unaffected. It makes the internet run just that much faster than before, and puts the control in your hands.
Needless to say, the ISPs don’t want you doing this, and they don’t approve, but in general, you can do so without their involvement and for the most part, they are entirely unaware that it’s happened.
Let me know what situation you’re in and we can probably devise a solution to the garbage ISP modem issue. Frankly, the fiber modem delete is my favorite.