AC66U with Shibby can't connect as WiFi Client with encryption enabled

I got Shibby's 112 firmware flashed to my brand-new AC66U last night, but I had difficulty getting it to connect as a wireless client to my existing network (E3000, downstairs) from an upstairs room. When I looked at the signal quality, it was typically 0, but would spike to between ~4 and ~13. I compared that to my WRT54GL, also configured as a wireless client of the same network and sitting in nearly the same spot, which was getting a solid/steady 20 to 21.

I tried, with no luck:
- Re-aiming...

AC66U with Shibby can't connect as WiFi Client with encryption enabled