For anyone who is reading this... while the method described by @robert-hh works to load the SiPy firmware into the board, it does not perform Sigfox activation which is necessary to connect to the Sigfox network.
If you have this issue, please do the following:
Note the MAC address of your board:import machine,binascii binascii.hexlify(machine.unique_id())
Email firstname.lastname@example.org and include the output of the above commands.
We will respond to your request within 24h on weekdays (often much sooner especially if you write to us during CET working hours).