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.
Thanks. I have an ISP provided media converter with my own router behind that, using the correct VLAN was enough to get it working. I thought those media converters were pretty dumb devices but it seems they are not.
FWIW, the incumbent ISP in Switzerland, Swisscom, tried to roll out XGS-PON but our "Internode", Init7, fought them in court on the grounds that it was anticompetitive, since it locks every provider into a single technology. They won.
Now customers can choose. Nearly every ISP chooses the easy way and has the customer connect through Swisscom's XGS-PON but Init7 in particular has instead built out their own routers in POPs around Switzerland so that customers can have a physical fibre directly to their network. It's just plain ethernet with DHCP so you can use whatever equipment you want. It's also allowed Init7 to do something none of the other providers can do: offer 25Gbps symmetric service at no extra cost (beyond a one-off installation cost for the more expensive SFP modules).