The One Plus 5T came out of the box with Android 7.1, and 8 is available. It is a flagship class phone from late 2017. Nexon just obsoleted Android 4.something, which was 3+ years old. So whatever his problem is, it has nothing to do with his version of android or the capabilities of his phone.
LordAnubis
Tinsoldier asked a reasonable followup question, to which the answer was no, this phone is not running an old OS.
Borck was helpful, in pointing out that the phone, which is an upgraded version of the one he has, should support the game.
Nexon has enough trouble with clueless technical support responses already. Having forum users respond by blaming the user is not helpful.