Updated marshmallows even has issues when connect to internet

[hdtbsaa]

Model: ZE551ML
RAM: 4G
Firmware/APP Version: 6.0.1
Frequency of Occurrence: All the time
Rooted: No
APP Name: Internet connection

My phone connect to internet via wifi and mobile data. But is always very slow in connection and google searches, app internet using, picture loading dose very low, or drop and i receive that no internet connection!!!! Recently i updated firmware to its latest version: 4.21.223_20161216 but after update apps the problem occurs again. I try safe mode too and every thing is ok in safe mode. But after reboot the problem exists. I think there is an app or service that leads system to that problem but i couldent find it. Can you help me? Excuse me due to my poor english:)

[ServiceH_ASUS]

Hi hdtbsaa Good day Can you help me confirm that phone is up to date? The latest version for ZE551ML is WW-4.21.40.352. In safe mode, only the factory pre-installed applications will be loaded.Therefore, please help to check the third party app which could influence the problem. Thank you:)