Samsung S5 & pre-installed Message+ not working properly

I live in a rural area with limited cell service.  I have wiFi at home.  I was beyond excited to see the phone came preinstalled with this app as I didnt realize it was an option.  I use that app exclusively for texting.  The problem is, when I am on wiFi it doesnt always send or receive messages.  I have to resend same messages up to 20 times and I receive the same message several times occasionally.  When I look in settings and registerd devices it doesnt have anything listed, on web app or mobile app.  I uninstalled yesterday and reinstalled thinking that may fix it.  I had to keep hitting subscribe and it would show starting service but never did anything.  A few hours later I received about 15 texts with a PIN number.  I can not find anywhere to enter those PIN's.

Someone please help!!

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