Things to check include bad connections in the power circuit (inside the outlets); some appliances can put electrical noise conditions on the wiring, ones with motors such as washing machines, blenders, refrigerators, etc (try running these through surge protectors if they are in your apartment), and also items with switch-mode power supplies (phone or tablet chargers).

You won't get 500Mbps. Whatever it says on the box that the adapters came in. That 1200 Mbps is a theoretical maximum speed. Users of powerline adapters, whatever the speed boasted by the maker, are lucky to get 200 Mbps, and most get much less. However, things to check include bad connections in the power circuit (inside the outlets); some appliances can put electrical noise conditions on the wiring, ones with motors such as washing machines, blenders, refrigerators, etc (try running these through surge protectors if they are in your apartment), and also items with switch-mode power supplies (phone or tablet chargers).