Once again thank you for your detailed answer. Therefore, If I apply the "overlay-802154.conf" overlay config then I will have the 6lowpan stack?
Yes, the OpenThread is really a nice choice.
Also, I am going to check the module nrf52840_pca10056 although doesn't support hardware acceleration as I can see. I would like to find a module which can support hardware acceleration for ECC in DTLS (not only pre-shared key). For now, as I can see in the tls_credential.h file the DTLS is using pre-shared key (PSK) only.