Q:

How do I set up a second 5GHz N-Only network with DD-WRT?

I have a Linksys WRT610N v2 running DD-WRT v24-sp2 (mega) - build 14896. I'm trying to set up two access points on the device. One 802.11g network for most of the computers and wireless devices in my house, and one 802.11n AP for a computer which is farther away.  
I can get the first AP (wl0) working with the following settings:
Wireless mode: AP
Wireless network mode: NG mixed
Channel: Auto
Width: 40Mhz
Control Channel: upper
Ack timing: 2000
Network config: bridged  
But the second AP (wl1) doesn't seem to work. At one point it was visible on my Mac Pro (the machine farther away) but when I connected to it, it just timed out. These are the current settings (which aren't showing the SSID):
Wireless mode: AP
Wireless network mode: N-Only (5 GHz)
Channel: 56 - 5.280 GHz
Width: 40 MHz
Control Channel: upper
Ack timing: 2000
Network config: bridged  
Both have security mode set to WPA2 Personal, with AES. 
Anyone have any experience with setting up DD-WRT and this router?

A:

I'm noticing a few problems with your planned setup. Some of them may just be problems with DD-WRT's UI (which I'm not familiar with), but I'm just going to list them all out here.

There's no value in turning off 802.11n in the 2.4GHz band. There seems to be a myth that you should for some reason turn off 802.11n in 2.4GHz if you only expect B/G devices to use it, but that's a myth and I have no idea where it comes from. On the contrary, you should leave it on so that your dual-band N devices can choose to do N in 2.4GHz when it suits their needs. For example, you can often get better data rates at longer ranges with 2.4GHz, so which it's better to use 5GHz when you're close, it's often better to use 2.4GHz when you're farther away. I note that your actual configuration you pasted in shows that you weren't trying to disable N in 2.4GHz, even though that seems to differ from what you said you were trying to do in the previous paragraph, so maybe I misunderstood what you were saying in that paragraph.
For 2.4GHz, why does it just say NG-mixed instead of "NGB-mixed"? Leaving the old B rates enabled for clients that choose to use them is a good idea. Sometimes you can get 1mbps connectivity out at the edge of range where the lowest G rate (6mbps) would fail you. Leaving B rates enabled for the sake of even super-modern clients that choose to use them is usually a good idea, especially if you care about range, which it seems like you do.
Again for 2.4GHz, why does it say Width: 40MHz? The 802.11n spec has no 40MHz-only mode. It has a 20MHz-only mode and a 20/40 automatic mode. I don't know whether this setting means standards-compliant 20/40 or means some nonstandard 40MHz-only mode. Also note that using 40MHz-wide channels in 2.4GHz takes up an awful lot of the band, and doesn't leave much room for other users of the band, such as your Bluetooth gear, Wii remotes, etc.  It's considered "good neighbor" policy to only use 20MHz-wide channels in 2.4GHz.
Again for 2.4GHz, why does it say Channel: Auto, Width: 40MHz, and Control Channel: upper? Shouldn't either the control channel or the width be auto in case it wants to pick channels 1-4, where you can't have the control channel above the extension channel? Or does this combination of settings just rule out channels 1-4 all together? Or does it automatically switch you to 20MHz mode if it wants to choose channels 1-4? It seems unclear from what you've reported here.
For 5GHz, why N-only mode? You should leave 802.11a enabled if you want the best rates at range, so that your Mac Pro or other machines can choose to use those legacy rates that work at longer distances than the N-only rates do. Maybe your Mac Pro is far enough away that it can't do N rates reliably, and it would do better if it had the option of doing A rates. For that matter, it would probably do even better if you used the same, non-hidden SSID on both networks, so that your Mac Pro could choose to use the 2.4GHz network when that was a better choice. I understand the desire to second-guess your devices' band choices, but if you didn't manage to make it work while second-guessing, maybe it's time to see if they work for themselves when you aren't second-guessing.
In 5GHz, why is it allowing you to manually pick a DFS (radar) channel? I see from your profile that you're in London. The UK follows the European (ETSI) RF regulations, and I believe the ETSI regulations require that implementations that allow use of DFS channels must not allow you to specify the channel. Implementations that want to be able to use DFS channels can only do so if you leave channel selection on "Auto", and at every boot/reset time they must randomly choose a channel, spend 60 seconds only listening to detect radar pulses on that channel, and only start using the channel if they didn't detect radar in that 60 seconds. So I guess it's possible that this channel didn't work because it detected radar pulses on that channel. You'll probably be better off leaving this channel selection on Auto.

