I could never break the 300mbps barrier on the 5GHz band even though I have a 500mbps subscription.

I have an Asus RT-AC1200G+ router that is capable of 867mbps on 802.11ac. I could only achieve 500mbps using a lan cable attached to my laptop.

I've tried running speed tests on multiple devices but all of them give out the same results. Why is that?