Finding a Netflix IPA (application package) for iOS 9.3.5 is difficult because Netflix officially supports iOS 12 or later
When you tap the cloud icon to download, iOS will detect your old firmware and ask if you want to download the "Last Compatible Version" Requirement: netflix ipa for ios 9.3.5
He logged into his Apple ID on his friend’s iPhone 15, "bought" the free Netflix app, and returned to the iPad. Finding a Netflix IPA (application package) for iOS 9
If the App Store doesn't work, another option is to use iTunes on a computer: "bought" the free Netflix app