Asus RT-AC86U not getting maximum speeds on gigabit fibre

Hi folks,

Wondering if you're able to help. Recently upgraded from the ISP-provided router for our gigabit fibre to the Asus RT-AC86U.

With the default settings, I'm only getting about 300/450 mbps download/upload speeds on both wireless and wired (Cat 5e) connections for a range of devices - Windows 10, MAC OS X, iOS.

I expected to get at least 650-750 download speeds at worst when I bought this so was wondering if it's my router configuration capping it somehow?

Thanks in advance!