We checked the wireshark logs and it seems that after Server Hello Done , the client is not able to proceed. I reproduced the DTLS handshake with Linux client and found that after this step, client then sends the whole Certificate to the server which is over 540 bytes. Now, do you know if there is any buffer limit on the size of data that you can send through cellular interface, something which is limiting it to less than, say 512. Because I can see some traces of IP fragmentation in the pcap logs but they are not originating from client IP but may be some gateway in between. That can be starting point to check the size limit for the data that can be sent or received through cellular modem , or is there some limit that you might have put on your porting layer ?