Aren't "CDMA" carriers hybrids these days: CDMA-based 3G but GSM-based 4G (LTE), which is why they now have SIM cards. But not being American (and CDMA being largely restricted to North America) I don't have experience of how that works in practice. We also don't know for sure what model S8+ this is, since we're not told which carrier it was originally sold on, just that it was used with ATT. But while the latest Galaxy S handsets should be fully cross-compatible one of this age may not be. For example if it wasn't a 955P (Sprint) or 955V (Verizon) it won't be compatible with the 3G bands used by a Verizon-using VMO, but may have 4G compatibility depending on what bands Xfinity/Verizon use in that area. In any case it would be worth checking the compatibility of the particular model.
The other thing I don't know is whether Xfinity require some sort of handset registration? It isn't technically necessary when the subscriber ID is handled by the SIM rather than an IMEI/MEID, but since CDMA users used to have to register their handset I can't rule out Xfinity still requiring this and only allowing handsets whose ID's have been registered with them. I'm not saying that is the case - my understanding is that VZW don't do this, for example - just that it would be worth checking that they don't put any silly barrier like that in the way (silly since that would cost them money to do something that's not necessary any more).
It's a long time since I handled a locked handset(*), but I thought they usually put a message on the screen if they were incompatible due to a SIM lock. Can't guarantee that a US carrier handset would behave that way though, as I've no experience with them.
(*) Here in the UK handset locking is finally being outlawed and no new locked handsets should be sold after the end of this year. A number of our carriers dropped it years ago, and our biggest independent phone seller has only ever sold unlocked phones even on carrier contract, so it's always been easy to avoid, but I'll be glad to see the back of it anyway.