EDIT: Disregard, I missed the part you did plug straight into your end device...I'd double check CLI and ensure offloading is enabled.
Have you tried testing the speed straight from the ONT device? When I had FTTH, I could plug straight into my laptop, taking the router out of the equation, to test internet speed. It would be momentarily to test the link, but at least you would be able to see if your ONT is actually offering a true 1 gig circuit.