StraightTalk S8 or S9 work with CDMA sim?

[M0mm3ra3ra3]

So our area has spotty service with anything other than Verizon towers. Checking with straight talk last night they said the S8 and S9 only work with AT&T. I'm confused because I thought those phones were "unlocked" and not strapped to a certain carrier. I have a CDMA sim right now from my S7, I'm wondering should I look to swappa for a Verizon unlocked or regular Unlocked phone?? I just want to make sure I'm going to have service before I spend the money on this phone. Thanks in advance.