Ok I found the solution
At home, when I use TP link C1200 with wireless N. No matter I set, it can't access and get ip. Even though reservation, auto ip, or static ip. It will solved only when took SD card from it. Then 942L will quickly work with Wifi.
I try another with TP link router wireless model TL-WR843ND . Only clue is set wireless region to "USA" that it ! problem solve though I live in Thailand. 942L quickly connect to wifi without problem. When I change region back to Thailand , it can't connect again till remove SD card. So, this proof problem originate from this!
Because TL-WR843ND has wisp function so, I attach them to my current wifi and separate web cam ip to TL-WR843ND Wifi. Now the problem gone. So, problem come from wifi setting in router. I don't want to waste time to find how to do in C1200 (because I had tried many ways on that around 5 hour, because it has no region setting on it) just wanna tell u guy for the solution.