So I bought a refurb 1-Day Galaxy S7 for my wife, which was an unlocked SM-G930U (ex Verizon), and it works excellently with Spark. It spent a huge amount of updating as soon as we turned it on.  Happy.

So I bought another for me, and this one turned out to be an unlocked SM-G930P (Sprint).  It too works well on Spark, but -- it has not updated at all from the original software, and pressing Settings - System updates does absolutely nothing at all.
There's nothing in About Phone concerning updates either. 

I'd very much like to be able to get the system and security updates for obvious reasons.  I suspect that I *would* get these if I was on the Sprint network.
So, you people who know, what do you suggest?

If rooting to stock android is the answer, what stock ROM would you suggest would be best?  Searching the internet only confuses me...and I don't want to brick it.  Links would be appreciated.

Thanks for any help.  But please don't tell me that I was an idiot, I may know this.