Plug a computer directly to the arris modem by ethernet..
Do a speed test.
You will never get speeds faster than this combo.
If the airport extreme is working as main router I am amazed you got the speed up to 700mbps as it is NOT gigabit router.. it has gigabit ports.. and will usually handle up to around 500Mbps .. no faster.
I've had the service tech out to my house and the gadget that he tests the network with only goes to 750 mb/s so it is hard to know what *really* is happening.
If they send service tech out with a tester that only goes to 750Mbps that tells me they expect to never reach gigabit.
Your test with a single computer plugged directly to the modem is the same thing.. if it reaches speeds of 700Mbps then that is your max speed. The ISP simply does not have the capacity to route faster than that to you and everyone else on your run who opted for gigabit.
If the computer does in fact reach full gigabit download speed.. (or full gigabit adding upload and download.. which is one way ISP can cheat with gigabit service). Then you need a much more powerful router.. the airport is not in the running.
Nor can you achieve gigabit actual throughput over wireless on an airport. You would need to buy the very latest AC3100 products with external 3100 receiver plugged into a port capable of that speed. No Mac is made with any wireless card greater than AC1300 (real world throughput on a good day.. around 600Mbps).