Reading the guide will help.
MVNO (Mobile Virtual Network Operator - a provider who buys service wholesale from a major mobile network operator and rebundles it into cheaper offerings than the mobile network operator frequently offers) - Consumer Cellular and Republic Wireless classify as MVNOs. AT&T and T-Mobile would classify as an MNO (Mobile Network Operator).
MMS APN - Don't worry about this too much, it's a phone setting. What matters about this in your case with an iPhone, however, is due to Apple phones specifically. Using an MVNO with a phone requires custom settings in the phone to access the internet and to receive group and multimedia messages. Apple (and this is one of my major beefs with them) has locked out their customers from accessing these settings on the AT&T network, and forbidden even backdoor reconfiguration options for the MMS settings on these phones. This restriction means that Apple gets to dictate at least in part which providers you can take your phone to for usage, unless you want a very key core functionality of the phone itself integrated into iMessage to completely break, preventing you from getting any multimedia messages, group texts, or messages in excess of 155 characters from any of your contacts who don't use iPhones themselves. As such, there's a very limited selection of "approved" AT&T MVNOs that Apple "permits" you to use without this functionality breaking. That short list is Consumer Cellular, Cricket, and Straight Talk.
Two of those three options have unsavory legal contracts and sub-par customer service, which is why I don't include them in the guide. It's also why I pointed you to Consumer Cellular. I tend to focus on companies that actually treat their customers with some respect and dignity instead of just chasing after the cheapest options, as supporting companies that treat their customers well tends to result in a far more pleasant experience, and preserves better options in the long run in a market that's incredibly cutthroat.
