Sorry.. I will try to clarify.
-cable modem with router, standard supplied by service provider.
-i believe ac66 and 1750 are the same..
https://www.asus.com/us/Networking/RT-AC66U-B1/
-in my previous posts if I said "cable" I meant the internet connection type. If you see "Ethernet cable" or "wired" I meant Ethernet cable.
-cable modem is not in bridge mode.
-i tested Dsl + Asus with cable wifi on and off and even with the cable modem unplugged completely. It made no difference in the results.
-each test specifies what is connected to what. In case one (good) cable modem is connected to Asus router. Laptop is connected to either the cable modem or Asus with an Ethernet cable or wifi 2.4ghz as specified in the various tests conducted with cable modem hooked up to asus. In case 2 (bad) cable modem is out of the picture and Dsl modem (bridge mode) is connected to the Asus. Each of the tests conducted in case one were repeated with Dsl as internet source.
-internet speed is expected to vary with cable internet, hence ranges were posted over several speed tests. In the case of Dsl the speed is expected to be constant and is constant (and full as promised by ISP) if connected with Ethernet cable through both Dsl modem and Asus (tested separately by plugging in laptop with a wire). Yes, i guess it is weird that the WiFi speed varies with dsl.. not sure if that is expected or not with Dsl, I have had cable all my life till now.
-when the tests were conducted no non standard things. No torrents or VPN. Now (shortly before my last post) I have configured open vpn client to selectively route one device (not the laptop) through a vpn service. So, this had no bearing on the information I posted as it was not set up at that time.
Hope that clears things up! Let me know if something is not clear and I will try to explain it better.