BG95-M3 Initial attach with IMSI

Hello,

We are using the BG95-M3 to run some NB-IoT connectivity tests. We would like to reproduce an EPS initial attach with IMSI, but our terminal keeps sending the attach request with GUTI. We also tried to reboot the module via AT command and manually, but the new attach request is always made with GUTI.
What are the conditions for the UE to send an attach request with IMSI? How can we reproduce an initial attach with IMSI?

Best regards,
Giovanni