Please tell us some basic information before asking for help:
Model Name: ASUS_I005DA (WW_I005D)
Firmware Version: 18.0840.2109.176
Rooted or not: Not
Frequency of Occurrence: All the time
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I recently had a problem with my signal on Rog Phone 5 and I think I found the cause of the problem.
I think "Google Carrier Services" is the problem. So find the app in google play store and uninstall it and make sure that it doesn't auto-update.
Let know if you have this problem and it worked