Ok, So something is seriously wrong with your connection. That's actually good news as its so bad that the NBNCo have to do something about it. FTTN is unfortunately the worst NBN Technology type and often has issues with line speed and dropouts.
At this stage you need to contact Optus. Run through some diagnostics with them (they should be able to confirm you are on the NBN50 pack and also tell you the max speed your line is rated at. If they can't improve the speed then they will arrange the NBNCo to come out and check the line locally.
How is the copper phone line leading into the house? All in good condition? No branches off to other outlet points?