G900V Reactivation Lock Not Disabling

[Osama6756]

I recently bought a used Galaxy S5. The seller provided me with the account info and now I do know the email and pass of that Samsung account but I want to create and link my own account to the phone. For that, I go to the security settings and click on the reactivation lock option to uncheck it. After inputting the email and password, the phone shows a "Please Wait" dialogue and after a while, returns back to settings. The reactivation lock is unchecked for a real small interval, like a single frame you can say and it's really hard to notice. Then it goes back on and it's as if nothing even changed. I really need help with this cuz I feel like the guy may want to change the account credentials sooner or later.