[Samsung Galaxy S8 Questions and Answers] ChazzMatt:
If you bought it from Sprint (or even just used it on Sprint), then you have a Snapdragon 835 S8. Only the Snapdragon variants have CDMA bands for Sprint and Verizon. Only the Snapdragon S8/S8+ variants are sold by carriers in North America -- so they can be cross compatible with Sprint/Verizon/AT&T/T-mobile/Canadian carriers. That's why whoever told you this nonsense is so ignorant.

If you had IMPORTED an Exynos variant from like England, then you would be limited to AT&T or T-mobile. But you have the "universal" Snapdragon model. Now the Exynos variant has an easily unlocked bootloader, which is a reason people import them, but that's another topic altogether.

As long as your Snapdragon 835 S8 is SIM unlocked (you have paid for it in full, and Sprint says they have unlocked it), you can take it to any other carrier or even an MVNO (like Straight Talk) and use it.

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If you used it on Sprint, you have Snapdragon 835. Only the Snapdragon S8/S8+ variants have CDMA bands for Sprint and Verizon, in addition to LTE bands.

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Since you phone is paid off, when you go to AT&T, try AT&T Prepaid (formerly called GoPhone). It's really for people who already own their phones.

Same high speed data off the same low-lag data servers used by "contract"/postpaid AT&T customers (with one exception, which I'll warn you about),

same in-store AT&T customer service as contract/postpaid AT&T customers,

same AT&T SIM card as contract/pospaid service. (The store may have SIM cards labelled "AT&T Prepaid", but underneath it's the same AT&T SIM card. If they try to charge you for one, tell them you want the FREE AT&T SIM card. It's the exact same thing.)

Make sure you go to an AT&T corporate store, not a "franchise" store. At the AT&T corporate store they will even give you free AT&T SIM card to use for AT&T Prepaid. (Franchise stores will try to sell you SIM cards.)

The advantage is AT&T Prepaid is much less expensive service than "postpaid" AT&T (contract service). Since you already own your phone in full, all you need to pay for is monthly service. You don't need to pay extra each month so you can get a "free" upgrade to a new phone in 2 or 3 years. (Not free, you are really paying for it with extra fees each month.)

You would want the $45/6GB plan on AT&T Prepaid. Do not fall for the $60 "unlimited" plan. The $45/6GB plan gives you plenty of UNCAPPED high speed data/unlimited calls/unlimited texts. Whereas the $60 plan caps your data speeds at 3mbps to compensate for it being "unlimited". AT&T Prepaid uses same data servers as their AT&T postpaid customers and I get ping speeds often less than 30ms, most often 26-29ms, which is fantastic low lag speed. (Lower pings are better, lower ping speed = less lag). If you go to even a service like Cricket (owned by AT&T, but you won't see any AT&T logos anywhere), they use proxy server farms and you often get triple digit ping speeds (high lag). This is also why Cricket has to use a DIFFERENT SIM card from regular AT&T. Higher ping speeds make it seems like your download speeds are much slower than they are.. due to LAG. Plus Cricket caps ALL download speeds at 8mbps. Until AT&T Prepaid (formerly called GoPhone) introduced their $60 "unlimited" plan, ALL their plans were uncapped high speed data. They didn't cap download speed at all. But now they have started capping speeds with the $60 "unlimited" plan. So STAY AWAY FROM THAT.

With the $45/6GB plan, if you autopay each month, you can save $5 and only pay $40 per month. So, $40 per month for 6GB data, with UNCAPPED low lag data straight from AT&T. Don't let them charge you for a SIM card! AT&T Prepaid uses the exact same SIM card as "regular' AT&T.

If you decide you don't like it, for whatever reason, you can switch to "regular" postpaid AT&T. But since I always pay for our phones in full, I like to save on monthly service. Although there are tons of MNVOs with prepaid service, they all use proxy servers with high lag (high ping speeds). Even AT&T's own Cricket is configured as an MVNO, to compete with MNVOs like Straight Talk. But AT&T Prepaid is not. It's "pure" AT&T, but at budget prices -- which AT&T uses to compete with actual carriers like T-mobile and Sprint.

You can "bring your own phone" , which in this case is your fully paid for Galaxy S8.