I have been trying to add a cap2N to my network - The last time I did this on my RB2011 it was easy and fast - I followed this tutorial - I thought it was really an excellent tutorial to be honest -
Then I purchased the RB4011 with both 2.4 and 5Ghz AP's - because I upgraded my internet to Fiber and wanted more through put -
BOth 2.4 and 5 Ghz Ap's are working perfectly on on the RB4011 - speeds and feeds are normal.
When I added the Cap2n I restored it to factory and then set it to Cap in quickest and followed the tutorial again...
I set my country code to Canada ( also tried US) - still get the same error on the 4011 No supported channel. You can see I am using a standard supported frequency for 2.4 at 2412 and 2442 which worked fine on the 2011- i also tried 2472
used this post as a reference point to resolve this --