WRT1900AC and Macbook Pro Retina - refuses to connect with Wireless N

Question

I have the first generation Macbook Pro Retina, and just recently upgraded my D-Link D-655 for the Linksys WRT1900AC.  I also put the router on the first floor (the D-Link was in the basement) because there were some huge holes in wireless coverage I hoped to fix.
To my dismay, the thing will not negotiate a connection with ANY wireless N device as wireless N.  When I set it to wireless-N only on 5 GHz, nothing will connect to it, and same with 2.4 GHz.  My macbook's system report shows the network as being Wireless G on the 2.4 band and wireless A on the 5 GHz.
I have 50 MBPs internet here, and when I plug directly into the router, I get that.  However, I am maxing out at 20 MBPs over wireless with both my macbook and my Samsung Galaxy S4.
I'm about at wits end, my next step is to return this thing.  I have been on hold with support for 20 minutes, which is freakin eerie because they have no hold music or talking so it's just dead silent.
This morning I spent an hour on a live chat with support, and nothing she had me do made it better, and in fact any changes made it drop from 20-21 MBPs down to 16-17 maximum.
On her advice, I re-installed the firmware, downloading from online, and this had absolutely no effect.  Any other ideas to try?  I get these crappy connection speeds sitting 3 feet away from it.  I live in a house, so no massive interference from neighbors (only 2 other wifi networks are visible)
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Answer 1

let me be clear here: I had the same slow speed with both my mac and my galaxy s4, it is not a mac problem.  I had disabled media prioritization and the WMM thing in the settings for media prioritization.  Yes I am sure I am connecting to the right SSID.  The old router is DIR-655.  Part of the reason I wanted to upgrade is it disconnected my galaxy s4 every 2 minutes (and many people have had that problem, a quick google search verified that the router just doesn't like galaxy s4).  OS X 10.7.5 but as I said my Galaxy has the same issue - it does not connect as wireless N, and when I make the router Wireless N only, not one of the computers in the house can connect to it.  That includes a Mac Mini, Galaxy S4, Macbook Air, Macbook Pro Retina, and a Chromecast (which only works on the 2.4 band, but same issue there).