Note that almost any menu item may be protected by password. It's matter of configuration.
Factory reset fixed the issue, but it revert pack after some time.
It may mean that there is Provisioning_rule configured so remote configuration gets reloaded. Reboots every once per day may be triggered by periodic re-provisioning as well.
Also note that 7.5.5b is not the latest firmware for this model. And it should be noted that direct upgrade from 7.4.3 to 7.5.5b is not possible. 7.5.2b must be used as intermediate step.
Note that remote provisioning may not fit well to upgraded firmware and there may be issues related to it. And upgrade to 7.5.2b is irreversible. If the upgrade will be incompatible with such particular provider then there is no official way to return to 7.4.3
So current configuration of the phone needs to be analyzed and further steps carefully watched.
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