If you want standalone messaging, do not install the main Messenger app for iOS 9.3.5 (it is equally ancient and glitchy). Instead, install —a 5MB app designed for emerging markets that still runs perfectly on iOS 9.3.5.
Sideloading a free Apple ID certificate via Cydia Impactor comes with a massive catch: After 7 days, the Facebook app will crash upon opening. facebook ipa for ios 9.3.5
Use Sideloadly to sign the IPA with your Apple ID and install it over USB. If you want standalone messaging, do not install