jonb: You will be having a similar install to me. Fibre cable to ONT in the garage. Then Cat 6 cable under house to Router in lounge. The phone port in the ONT is connected to where the old phone cable entered the property (where your current master filter is located) to switch house phone wiring from copper to fibre.
Cat6 part from Router to ONT may be at your own cost. You could always just temporarily have the HG858 next to ONT to allow a successful install to be ticked off, and then sort the eventual location of the router yourself afterwards.
1. Where is your FTB?
2. Did the installer route Cat6 to your router?