You can find the downgrade firmware here- https://rog.asus.com/phones/rog-phone-5s-model/helpdesk_download/?model=rog%20phone%205s Image version 18.1220.2201.192 for WW or 18.1220.2201.189 for CN version.
Before you start the downgrade, please make sure all pictures, important data finished the backup.
The downgrade process will erase all data to be a new system. ASUS company would not be responsible for the risk of data loss.
Steps of Update:
1. Download the firmware to internal storage.
2. Remove Google Account.
3. Pull down the quick setting list.
4. The system will pop up a message which detected a firmware update info.