I got a SWARM M138 Modem kit from Sparkfun and I will share my experience before you decide to buy yours too.
First thing you need to know: it is not an “Always ON” communication channel. Actually the interval between the satellites passing over your head is around 3 hours. You can use the SWARM checker to see the right moment that the satellites will pass: https://kube.tools.swarm.space/pass-checker/
So it is not a device for “realtime” (online) tracking like SPOT Gen from Globalstar, but it is more affordable and could be used as a backup for emergency applications.
Fortunately Sparkfun created a nice Swarm M138 GUI to let you to test the modem just clicking in some buttons.
After connecting the modem you could see some booting messages:
Let’s to send a message:
Another important thing I noticed is that after the message is sent it will not appear immediately at SWARM hive: https://bumblebee.hive.swarm.space/messages
Probably the messages are transferred from a nanosat to another until it reach some ground station.
References:
https://learn.sparkfun.com/tutorials/satellite-transceiver-breakout—swarm-m138—hookup-guide/all
https://nootropicdesign.com/projectlab/