You’d need to be able to replicate whatever configuration the ISP provided device has, and they won’t give you that.

FTTH here in Australia is the same, you’re stuck using the network providers device, which just provides an Ethernet port, and a POTS port if you’re in to that sort of thing, with your LAN device connected behind it.

There was fierce lobbying back in the day (shout out to Simon Hackett / Internode) for our national broadband network to be simple dark fibre and that ISPs could build on top of that to provide innovation and differentiation.

Instead what we got was a bunch of ISPs that resell the National Broadband Network’s expensive wholesale plans with little in the way of either differentiation or innovation.

Edit to add: what the sibling comments said too.