You don't need to enable VoLTE. Insert the Jio (VoLTE enabled) in any slot. By default VoLTE will be active. If it is not, you can change the network preferences from Network option in settings.
5G technology is entirely new and still in testing face among telecom companies, device manufacturing companies, and software companies. Though rules of the 5G have been finalized, prototypes have yet not been finalized and phones in today's market will not be compatible with the 5G Technology.
Apple iPhone x does not support 5G and will not be compatible with it.
Apple iPhone never compromises in quality, so every time iPhone is better than the android phone... read more information why Are iPhones better than Androids mobiles?