Xfinity mobile charged $1000+ roaming overcharge

I am posting here because we haven't been given proper explanation why we should be charged over $1000 roaming while we were traveling in Japan!

Xfinity mobile charged us $1,000+ roaming fees for one of three phones when traveling in Japan although we had roaming OFF!  We had portable wifi devices from a local provider.  So, all data usages were strictly through wifi. 

For the two phones, roaming was OFF, global travel pass was NOT active, data usages were strictly through wifi. 

For the one that was charged roaming, Global Travel Pass was active for the first 6 days of the trip because the user (my son) was traveling separately and he didn't have a portable wifi device. During the 6 days, he had roaming ON in order to access data through travel pass. Later evening on Day 6, he reunited with us and was able to connect to the portable wifi device just like the other two phones. He turned roaming OFF and also removed travel pass since he no longer needed travel pass. Two days later, we received a text from Xfinity, notifying us that we were being charged for roaming fee $684.71!  Three hours later, another text from Xfinity came in that the roaming charge is now $922.38. Another three hours later, another text from Xfinity came in that roaming charge is now $1017.71!  Right after we received the first notification of $684.71, we checked his phone setting and confirmed that roaming had been OFF. So why should we be charged for roaming fees at all?

We immediately contacted Xfinity customer service but the agent could not help us even after spending over three hours. We were still traveling at the time, so dealing with the roaming charge was very stressful and frustrating.  Since then, we've contacted Xfinity multiple times through Xfinity app, phone calls to 888-936-4968, virtual customer assistance, and Xfinity store. They kept telling us different things every time we spoke. We are now told to go back to Xfinity store if we want to see data usage since the customer representative cannot share it with us. The rep told us that only way we can obtain data usage is if we go to a Xfinity store and take a screen shot on the store computer although we requested the information sent to us via mail or email. This is outrageous! We've already spent countless hours, trying to solve the issue, not to mention mental stress and financial stress that this has caused. Taking time to solve the issue out of our daily life is not something easy to do. It has been a month since then, it is yet to be fixed. Only "resolution" that we were offered was a payment plan!, Why should we sign up for a payment plan when we are disputing the charge and we think that the error is with Xfinity?

So far, they've told us that

1. Travel pass shouldn't have been removed if we were still overseas. With "By the GIG" plan, data is charged $0.30/MB - We didn't need it since we used a portal wifi. Why should we have kept it on and pay $10/day?

2. Calls and texts use data because our plan is "By the Gig" plan which calls & texts use would be "Pay as. you go"- According to Xfinity website, calls and texts were included without any additional cost!

3. Even a phone is connected to wifi, data can get charged-  Doesn't make sense. We used a portable wifi for data usage while having roaming OFF.

Could anyone please explain

A. why only one phone was charged for roaming while all three phones had roaming OFF?

B. why roaming charges incurred when roaming was OFF and we could use app or internet browsing only when it was connected to wifi?

Did anyone have similar issue? If so, how did you get it solved? 

[Edited: "Soliciting"]