The Mayfly stations with 4G-LTE modules that we deployed for you earlier this year were all configured to only use AT&T as the provider, but since then we have tweaked the code to allow them to work with either AT&T or Verizon, whichever carrier has the strongest signal at your location. It still uses the same Hologram SIM card, it’s just a configuration that the Mayfly writes to the LTE Bee when you boot up the Mayfly. If you download the latest Modular Sensors libraries (0.23.16) from our Github repo and use an example sketch from that directory, then it will configure the module properly for either cell carrier. If you haven’t programmed a logger with ModularSensors yet, you’ll probably need a bunch of the other libraries as well, so it’s easiest to just grab everything from here: https://github.com/EnviroDIY/Libraries
Follow the instructions in the ReadMe that tells you how to properly download all of the files in the Libraries repo. And if you’re using the Arduino IDE and you already have some of the folders that are contained in the EnviroDIY Libraries repo (which is probably very likely), then you might end up with conflicts due to having multiple copies of similar libraries, so you might have to resolve those. Using something like PlatformIO as your IDE will solve the library conflicts, though it has a pretty steep learning curve if you’re new to programming.