Change Settings.Global.AUTO_TIME to always be 1
Since the phone is rooted, it would be possible for an Xposed module to achieve this by intercepting the call to Settings.Global.getInt and returning 1 regardless of the real value, but I don't know of a module that does this, so you'd have to write it yourself. You should consult the Xposed documentation to learn how; I'm afraid we can't help with programming problems on this site.