why is my phone behaving so weirdly????

[iMore Question]

few days before i bought a second hand iphone 6s probably unlocked one as it wasn't with bill. and i did a random check for the JAILBEAK CYDIA APP and couldnt find it. so after i purchased it i tried to update it using OTA but it kept checking and checking for updates and never returned the result. so i tried itunes. but when i plugged my iphone to itunes it showed that itunes couldn't backup because the iphone disconnected. still i downloaded update on itunes thinking i would update without backup but it didnt happen because when i tried to update it returned an error. so i thought to update using DFU mode but then i noticed a strange thing. the itunes showed different serial number and detected my 6s as iphone 5s and couldnt update as firmware was incompatible. being disappointed i thought to erase iphone but wen i clicked erase iphone for 2nd time it displayed a chinese or japanese option with 2 choices either of which i clicked but erase didnt happen. after that i resetted all the setting first and then erased the iphone which was successful. after sometime my iphone suddenly rebooted into safe mode showing that springboard app just crashed so mobile substrated saved the iphone bla bla. now i feel like my iphone is jailbroken but there is no cydia app in it. what must be the problem?

 

[Tartarus]

All I can think of is you bought an iPhone 5s in an iPhone 6s housing.
Open Settings in your iPhone, go to General > About and write down your Serial Number.

Then go to https://checkcoverage.apple.com/ and see what iPhone you have bought.