iOS update failed on iPhone 12 pro max

Hi I have an iPhone 12 pro max, it is running on ios 14.1. 

I haven't been able to do any update since I bought it last thanksgiving.

I tried during different time of the day, and on different days,

it keeps give error: "an error occurred downloading"

1. there are a lot of storage space on this phone.( more than 110GB free space)
2. I have no problem update my iPad to any new version from home. i also have no problem download any App from App store. So, there is not any wifi connection issue.
3. I have "Automatic Updates" on (both "download iOS updates" and "Install iOS Updates" are on)
4. I did "Reset Network Settings", it does not help
5. I did try update manually, got the same error.

could it be a device defect caused my issue? is there anything else to try?

thanks for the help.