I am having issues finding something specific. I am almost positive there is an iPhone that is able to do both GSM and CDMA technologies (universally unlocked). However I am having a hard time remembering which ones they were. This is not dual sim I'm talking about, but just a really powerful one sim slot. Any and all help is appreciated.
This article may help a little, but I'm not sure if some or all of the Verizon and Sprint iPhones need to be unlocked from a carrier before they can used on GSM carries, etc.
"The iPhone 7, 8, X, XS, and XR are a bit different. There are two versions of each phone:
The Verizon and Sprint variants have CDMA and GSM chips on the inside, and can be taken to any other carrier as long as the phone is unlocked.
The AT&T and T-Mobile variants, however, only come with a GSM chip. That means you can’t use an AT&T or T-Mobile iPhone on Verizon or Sprint, since those versions don’t have CDMA chips. (You can, however, take an AT&T iPhone to T-Mobile, or vice versa)."
If you go back to even older models:
All variants of the iPhone 6s work on both CDMA and GSM. The A1633 model is optimized for AT&T.
All variants of the iPhone 6 work on both CDMA and GSM. The A1586 model is optimized for Sprint. The A1533 for the other 3 major carriers works on Sprint but not well.
For the 5s, the A1533 originally issued for Verizon works on CDMA and GSM. This is the best one, because it happens to be unlocked out of the box. The A1456 originally issued for Sprint can also be made to work on GSM if unlocked by Sprint but works poorly on AT&T.
iPhone 6s or 6s plus is cheap and powerful, works with all carriers if the phone is unlocked. If you are interested in T-Mobile or its MVNOs, go with newer phones with LTE band 71 which will boost your T-Mobile reception dramatically.