Apple confirmed that it's aware of the issues and posted some suggestions on how to fix them:
Under certain conditions, iPad may not automatically rejoin a known Wi-Fi network after restart or waking from sleep. This can occur with some third-party Wi-Fi routers that are dual-band capable when:
- Using the same network name for each network
- Using different security settings for each network
If you encounter this issue, try the following:
Note: Always ensure that your Wi-Fi router firmware is up to date.
- Create separate Wi-Fi network names to identify each band. This can be done easily by appending one or more characters to the current network name. (Example: Add a G to the 802.11b/g network name and an N to the 802.11n network name.)
- Ensure that both networks use the same security type (WEP, WPA, WPA2, and so on)
- If the issue persists, reset your network settings using Settings > General > Reset > Reset Network Settings.
How to Fix the iPad's Wi-Fi Issues - Ipad wifi - Gizmodo
iPad: Does not automatically rejoin known Wi-Fi networks