Could you please read again my post? It's router itself. How can wifi card connect at 40MHz if the router don't work on 40MHz?
Anyway, thanks all for your reply.
I found a limitation from the release note of 1.76 firmware on Japanese site, even on US site, they don't mention about this...
Link
Maybe they're different...
? Limitations
1. If there is a nearby access point, double-speed mode (40MHz) can be set to
Normal mode automatically (20MHz) it may be.