Comment by lloeki

In France there's a long (as evidenced by the phpbb forum) active community of people doing that, the keyword is "box† bypass"

https://lafibre.info/routeurs/ (see "remplacer XXX par un routeur")

It's somewhat easy if you know network stuff (DNS, DHCP options, CIDR, routing, prefix delegation, etc...); if you somewhat know about those and can navigate some docs the hardest part is figuring out what the ISP expects in terms of magic bits so that it decides to:

a) hand you out v4 address (DHCP) / v6 prefix (RS)

b) allow routing (notably for v6 prefix, sometimes /64 may work OOTB but getting the /48 or /52 you're supposed to have doesn't route without proper RS DHCPv6 options)

c) unlock throttling (some throttle packets unless magic bits are sent in a specific way)

Well; that, and having the proper hardware (e.g obtaining an off-the-shelf 10-GPON SPF module compatible with what your ISP provides might range from very pricey to unobtainium). I'm lucky because my ISP chose a split ONT/router strategy and I just have to plug my own router in the GPON ONT over Ethernet.

† an ISP-provided router is just called a "box" - from "set-top box" - commercial names being "box" concatenated with the ISP initial or name: Bbox, freebox, livebox, neufbox/box SFR