I think it's fair to say the vast majority of users won't be able to pull that off. I doubt even half of them know what a router is, let alone that there are differences between models.
For those who know they can use their own modems, sufficient information must be available, but only a sliver of the people with AVM modems will have that kind of knowledge.
There is the class that fails at reading and adhering to IKEA instructions, yeah.
But this is something that ask the non-obvious things will get explained to you if you walk into a MediaMarkt to where modern routers are and queue in line for the area's sales person to get to you and tell you what to buy, and how to get your hands on the relevant access credentials/how to get the new one to connect to the ISP.
You're forgetting that most installs of non-ISP-provided moderns for residential Internet are set up by the tech person of the household who quite possibly never heard of what a NAS is and why they may want one.
Often the only paper manual thing in the box is literally the quick start guide that a motivated person who has what could be called "common sense" on treatment of/interaction with computing equipment. You know, the person who knows to check the plugs because they don't consider themselves above it but do know that it's one of if not the first thing they are asked if they can support.
For those who know they can use their own modems, sufficient information must be available, but only a sliver of the people with AVM modems will have that kind of knowledge.