Where can I complain about the product?

If you have specific bug, feature or hardware recommendation Apple does provide a feedback page here https://www.apple.com/feedback/ Though if you are experiencing a specific technical issue that you need addressed you can post your issue here to see if someone from the user community can help. If your iPad is still under the 12 month warranty or you have Apple Care coverage you can reach out to Apple Support by selecting “Support” at the top of this page then select the product then select Apple Support.

You need to search online for your nearest, “local” Apple store location and call contact phone numbers and call to make an appointment to have your Apple iPad diagnosed and options presented you.

Failing that,

You need to contact Apple support via the contact support link located at the upper right hand corner of every Webpage of these Apple support communities.

Use one or more of the support categories.

OR

by installing and using the Apple Support app found in the iOS App Store.

Tap the iOS App Store link below.

https://apps.apple.com/us/app/apple-support/id1130498044

OR

Call contact Apple via the support numbers found in the link bekow.

Contact - How to Contact Us - Apple

Tap the link below.

https://www.apple.com/contact/

Best of Luck to You!