@toxster 

Try the following suggestions if none of them resolve you issue please reach back out.

Since your iPad is an older device and can successfully connect to the IoT network, this suggests a compatibility issue with the main network's encryption settings. The Deco BE11000 may be using WPA3 encryption on the main network by default, which older devices like your iPad running iOS 10.3.3 don't support. The IoT network typically uses WPA2/WPA-PSK for better compatibility, which is why it works.

Here are some troubleshooting steps to try:

1) Change the encryption method: Go to the Deco APP > MORE > Wi-Fi settings, and change the main network encryption method to WPA2/WPA-PSK. This should provide better compatibility with your older iPad.

2) Disable Fast Roaming: Go to the Deco APP > MORE > Advanced > Fast Roaming, and disable it. Older devices sometimes have issues with this feature.

3) Check for special characters: Double check your Wi-Fi SSID and password, and avoid using special characters such as spaces, hash/pound signs (#), dollar signs ($), slashes (/ or \), percent signs (%), asterisks (*), at symbols (@), ampersands (&), brackets, non-English quotation marks, or any non-English letters or numbers.

4) Verify Access Control: Go to the Deco APP > MORE > Access Control (Allow List or Block List), and confirm the device isn't mistakenly blocked.

The encryption method change (step 1) is most likely to resolve your issue, as it addresses the compatibility gap between newer security standards and your older iOS device.