Have you tried power cycling the ONT, it worked for me when I went from 500 to 900 but it may have just been a coincidence.
The ONT is the fibre modem which you have connected to the hub probably with the red ended ethernet cable. Small white box with probably 3 green !ights
Just switch it off wait and then switch back on
Did that but still just over 500. Upload is averaging 115 which is about right I think.