So what is your network card? Is it AC, connection of 270/300 tells me N 2x2 card..
What speed did you expect? And for full AC you would need to set to 80mhz channels.
Also I would move your channel of 2.4 down to 1 since you have something else running at 11, might as well move away from that. As to not running 40 on 2.4 since it is only being small difference... Its a bit more than you think.. Seems you have nothing in the area so there should be no reason to not run it unless your card has issue with it, or you operate your wireless client in area of your wireless coverage where you don't have good strong signal. If you no other wireless in the area, and you have good signal there is no reason you should not be able to run 40 on your 2.4
But again I have to ask what you expect to see in real world speeds.. You do understand the numbers give are all marketing hype, for example the AC1750 listed for your router is both bands added together -- which clearly your client can only use 1 of them. And then even the numbers they give total raw bandwidth so pretty much /2 that for anything even close to what you might actually see
And then again that router is AC, if you don't have AC at your client its not going to do you any good.