Background Apps and Data usage international

I have been a loyal at&t customer since 2001. I went overseas for the holidays and was very disappointed to see my at&t bill $700 higher than my usual balance. We have been traveling overseas for the past 20 years, so as not to incur hefty charges my we simply put our phones on airplane mode and utilize wifi to do text messages, calls and use the internet. On my recent trip I  purchased the passport for one of my sons, as they were going to another island and I wanted to make sure that he could use his phone if needed. Upon receiving my enormous bill I immediately called at&t and the first phone agent whose name is  "Zach" based in Georgia,  told me that his manager approved a $500 credit specifically for the erroneous data roaming charge of $563.00, while my son was on board the airplane. My son's phone was on airplane mode, and was not utilizing cellular network. I was also charged for phone calls while on airplane mode and  using wifi. I do not know how it is possible to be charged when my phone is on airplane mode and cellular network is off. The second agent that handled my complaint was a "Rebecca" supervisor in Arkansas told me that she cannot help me and that there is no $500 credit reflected on my account. I told her that phone calls are recorded and they should be able to hear clearly that I was told that there would be a  $500 credit. The third agent I spoke to was a named Joe working from an offshore call center and was  the only one who empathized  and thanked me for my loyalty. He took the details of my complaint and told me that he would file a case on my behalf, and that I should check online the status of my account. He said it would take a maximum of 5 days to receive a decision. I am really disappointed in the initial customer service I received. My husband has Tmobile and he used his phone in the same manner we all did, and he did not get charged for anything.  I will switch to T mobile if I don't get a favorable resolution to this matter. This is the very first time I am getting charged for using my phone on airplane mode with cellular network turned off. My sons travelled to the same country last year july2018 and they used their phones  the same way and I did not get charged. If At&t simply changed the rules the least they could do is inform the customer. I don't think its right for at&t to charge when cellular network is off.  If I turn off my cellular network which is At&T how do they have a right to charge me!