Google, back in OS 5, removed old HTTPS ciphers due to security concerns. The old ISY-99i (firmware 3.3.10) only supported these old (insecure) HTTPS ciphers. As a result Android 5+ and iOS 9+ devices no longer can connect to firmware 3.3.10 over HTTPS.
You can still use HTTP but the recommended solution, if you think you'll be keeping the ISY/INSTEON or want to add Z-Wave to the mix, is to upgrade to the ISY-994i. If you contact UDI Sales and ask about their upgrade options for ISY-99i users, they can give you the upgrade details. Please contact firstname.lastname@example.org