Galaxy S6 edge - cannot disable roaming data notification warning

[Android Central Question]

I have Galaxy S6 edge with T-Mobile but spend most of my time in New Zealand rather than the US. But after the most recent software update (July 2017 from memory), tI cannot stop the phone giving me the 'roaming data notification warning' and 'yes' I have turned off mobile data and only use Wi-fi when I'm abroad. The warning comes up intermittently between 10 and 20 times per day and is very annoying.
Any solutions to this problem would be much appreciated