Who I have the same questioned this solution

Hello, my wife and I are traveling in Poland and trying to use foreign SIM cards in our phone. We're on a current contract plan and both have HTC One (M8)s, which are dual service/GSM compatible. My understanding was that all Verizon 4G phones are unlocked so I thought that meant we could just insert the new SIM card into the phone and it would work. But the phone just says it's not a Verizon SIM and the data won't work (although the carrier, Orange, was sending texts). I've tried Global mode and the GSM only mode and neither works.

I see similar questions in the forum, but can't find any others that actually answer the question about what to do to make it work. What am I missing?

Thanks in advance!

Dave