sim not supported

I have an iPhone 5s verizon version prepaid version, when I tried to use it overseas it gives me message sim not supported I contacted with verizon several times and they insist the phone is unlocked and they don't lock their phones then contacted with with Apple support according to them I should talk with verizon because they've nothing to do, after several times trying with both I got tired so I tried with third party unlock service after they reply to my message with the iPhone is unlocked and they give instructions which is the the same as it in Apple website nothing new happened so they refund my money, I already tried everything I found in Internet the phone now stuck-up with activation screen that happened after several times of DFU restore because in first I was able to use apps without sim card