Sometime FOTA just don't show.
If your phone is WW SKU (worldwide) then you have to update it manually, downloading the file here:
https://dlcdnets.asus.com/pub/ASUS/ZenFone/ZS620KL/UL-Z01R-WW-100.10.107.70-user.zip
Backup everything, remove Google account from "Accounts" under "Settings", making sure you know its password to access again when it's done.
The download already finished?
Go to your files manager app under your app drawer.
Select the option called "Downloads" and hold the file inside it until it shows some options.
You will then touch on "Move file", and select Internal Storage, then "OK".
Restart the phone and wait for a notification.
Accept and confirm everything and wait.
Restart when asked.
Enter your Google Account.
Then its done.
