Brief history of WiFi.
They were developing a standard on the 5GHz band, which they name 802.11a, but it took so long that the 802.11b standard for 2.4GHz was released at the same time and the 802.11b had a lot more support due its better range. Mean time the 802.11g on the 2.4GHz standard was also about to be released and it was backwards compatible with 802.11b devices, allowing a smooth upgrade transition, whereas 802.11a operates on a completely different range and is not compatible.
So then everyone crowded the 2.4GHz band (WiFi and Bluetooth), which caused a lot of interference, resulting in poor signal strength, etc. therefore they released 802.11n, which was the first dual-band standard. It was able to operate on both the 2.4GHz frequency to allow better range and the 5GHz frequency, which is faster but at a shorter distance.
And so all was going well, but there's still the really slow data rate on the 802.11n (about 15MB/s real-world on a good day in my case) which was shared across all connected devices. So they decided to release the 802.11ac standard, which allowed one to go really fast on the 5GHz band, which was great.
WiFi works with "streams", the more streams you have with more frequency that you allocate, the more data can be transferred at a time.
When a provider advertises that it's AC750, they're adding both the 2.4GHz band and the 5GHz band, as you can make them use the same SSID and if the device supports both bands, you can "use them as one", though it's the theoretical limit and not the real-world speed.
Check
here under advertised, someone was nice enough to make a table.
I have the Archer D5, AC1200, "AC1200 300 2 streams @ MCS 7 867 2 streams @ MCS 9", I get about 75MB/s real-world on the 5GHz band when copying stuff when I separated (made them different SSID) to test it, about 90MB/s when I combine them under the same SSID, with the 5GHz stream dropping off sharply in signal strength through a wall, but still good throughput at about 50MB/s and then being non-existent a wall further.
