Security Management for Wi-SUN network through Nanostack

I have a router with Wi-SUN router firmware flashed in it (mbed with nanostack), and I want to connect to another Border Router (non-ST and non-mbed). I can sniff the frames on both sides so I know I have security alignement issue.

My router send a PAN Advertisiment Solicit, which is received by the BR :

On the first PAN Advertisement, security is set to false (bit 3 of Frame Control Field, see of 802.15.4 2020). But the PAN Configuration from the BR comes with a security set to True, and we can see a MIC in the message :

The other frames have also security enabled, but anyway, I think that I don’t have security enabled on my router. How can I do to be able to handle MIC, and more generally to handle the frames received from the BR ?

Thank you very much for your help.