Xfinity mobile charged me a $8000 high data roaming fee!!!

Xfinity mobile charged me a $8000 high data roaming fee!!! I tried to call them to dispute the bill, but they told me that the charge was valid. This past November, I was traveling to Guam and stayed there for only one night, and in about 3 hours, they charged me about an $8000 data roaming fee. 

Most importantly, I think this was not my fault because: 1) Guam is part of the US territories, so I did not know I would be charged differently. 2) I did not receive the warning message about Guam being outside of the US and that they will charge me differently; 3) The data roaming option on my phone had been turned off when I was in Guam; 4) I had connected my phone to the wifi when I got to the Hotel; 5) I did not receive any warning that my phone was charged a very high data roaming fee.

First, I want to ask people here what I should do to dispute this super-high bill.

Second, I want to ask people here if Xfinity should charge their customers such a data roaming fee at all? After all, anyone here could become of such a consuming trap set by the company. Even though I tried very hard, I still could not avoid it. Please share what you think. I really appreciate it!  

I also asked them why they did not apply for the international travel pass automatically, and they said they could not do it. I said there are two choices: 1) pay the $8000 data roaming fee; 2) automatically apply for the international travel pass if people did not turn off their phones. Which one customers will choose?