I got it working again with everything, however yet again my MBA suddenly decided to not work on its wifi. It connects to wifi but not the internet.
1) I'm located in Western Europe.
2) I didn't do a factory update before or after the firmware update.
3) I have 20Mhz set on both routers just now.
4) I set one router to 1 and the other router to 6.
5) I've WPA-Personal, with WPA mode being WPA2 only and Cipher type AES on both routers.
6) There are 6 SSIDS in the area. 2 of them are my DIR-655 and one of them is my printer's one.
7) I've now turned off Short GI (as it was on). WLAN Partition is now on, "HT 20/40 Co-exhistance" - I'm not sure what this is... I've also turned off extra wireless protection.
WMM enable was already enabled.
I then rebooted the system and tried to connect using my mac book. It worked. I'll report back here when it decides not to work again in 2 days time.
I have DIR-655 Hardware version A3, Firmware 1.37NA (connected to my ISP and is wifi hotspot also)
And
DIR-655 Hardware version A4, Firmware 1.37NA. (used as a wifi hotspot only)