My modem is the Arris SB6190. My router is the Apple A1521.
On a speed test for a computer ethernet wired from the router, I am actually getting 700mpbs.
That is remarkable since the AirPort Extreme usually checks out at a top speed of 300-350 Mbps using IPv4. You must have IPv6 set up correctly on the AirPort Extreme to be able to record speeds that are roughly double the normal expected for the router.
I have 400 Mbps Internet service here which usually checks out at about 430-440 when I connect my Mac directly to the modem over Ethernet. When I connect my Mac to an AirPort Extreme A1521 connected to the modem, I can get maybe 320 Mbps on a good day. Usually, its right around 300-305.
And IPv6 is set up well.......the IPv6 tests check out at 10 of 10.
Remember that that AirPort Extreme was designed in 2012 and introduced in early 2013 at a time when average speeds for ISPs were much lower. 300 Mbps in 2013 was really fast.
As far as 5.0 WiFi, if you have a Mac, hold down the option key while you click not the WiFi menu at the top of the screen. Look for the Tx Rate, which is a reading of the potential speed of the connection between your Mac and the AirPort Extreme.
What reading do you get if the Mac is in close proximity to the AirPort Extreme with line-of-sight to the AirPort?
What reading do you get if the Mac is a room away so that the WiFi signal must pass through one wall?
What reading do you get if the Mac is located such that the WiFi signal must pass through two walls?
