What kind of distance are you trying to cover with your radios? There are a variety of Xbee options that will work for short distances (several hundred meters), but there are also models for transmitting over a kilometer or more. There are different models for different regions of the world because of frequency regulations and other issues, so I can’t point you directly to a specific model, but I’ve had good luck with the 900MHz USA version of the Pro radios and a small external antenna.
For cellular options, we’ve had really good success with using SODAQ’s GPRSbee (https://shop.sodaq.com/en/gprsbee.html). All you have to do is add a SIM card and plug the GPRSBee into the Mayfly. We’ve also successfully used WifiBee modules with the Mayfly.
SODAQ also sells a LoRaBee (https://shop.sodaq.com/en/lorabee-rn2483.html) module, but I have not used it yet and can’t offer any advice on that, but I think it should work fine with the Mayfly as well.