After few days on searching I believe the followings are what I found.
1. Nothing wrong with the update in fact for N 5G there is a big improvement, since iPad and mac are using 5GN the speeds are improved.
2. However, iPhone is using b/g/n 2.4 G which are normally over crowned with the other same channels from the neighboure. During the updated if ur previous setting was auto, the previous channel would be takenby others, and if someone sharing the channel will overcome yours, causing the problem
3. Not until I installed the wifi radar, mine was 11 but someone using the same one was more powerful than mine, therefore I forced to pick a channel that no one using
4. U can try to start from 1 to 13 and manually pick the best one