There is really only two ways ( issues ) that the speed provided isn’t what you pay for with FTTP , well ones that your provider or network provider are responsible for or in control of ,
the first is that you have been configured ( set up ) incorrectly from the beginning , basically if your service was incorrectly set up with the wrong speed profile, as you get 280Mb ish then potentially you may have been set up with a 300Mb service instead of 500Mb , or second there is congestion in the news ( that’s were other users are reducing the total available bandwidth so your throughput , not sync speed , suffers ) ,this second one is easy to test for , you simply test at a period when it’s reasonable to assume not may other users will be on-line , peak time is 6-9pm , if you test your speed outside these times , if the speed doest improve to the minimum guaranteed rate , then it’s not congestion.
It is however , in most cases like this , the customer’s own equipment, or unrealistic expectations of what their kit can deliver…if your tests are on a wireless device ( so iPad , iPhone etc ) then they are not testing the limits if the service , but the limits of the WiFi within your property, only testing with a wired device that is connected with an Ethernet cable can prove if the service is working within specifications or not .
TBH , it probably is , no ISP guarantees WiFi speed , there are too many environmental variables that are beyond your ISP control , once you prove it’s WiFi coverage and not a network problem you can explore WiFi extenders , access points etc
