BC660K-GL DTLS error


I am using a BC660K-GL NB-IoT monule to send data to an IoT platform (Thingsboard). It works fine, but I have decided to try the DTLS feauture of the module as well.
After configuring the module, I keep receiving +qsslopen: 0,0,-9 response when I want to open the ssl connection. (I loaded all the certificates, and private key). The cloud supports the DTLS, I have tested it with the linux dtls CLI, t worked well.

Do you have any idea what could cause this error?
Thank you for your help in advance.

AT+QSSLCFG=0,0,“dtls”,1 //Enable DTLS
AT+QSSLCFG=0,0,“seclevel”,2 //Perform server and client authentication
AT+QSSLCFG=0,0,“cacert” //Configure CA certificate
AT+QSSLCFG=0,0,“clientcert” //Configure client certificate
AT+QSSLCFG=0,0,“clientkey” //Configure client private key
AT+QSSLOPEN=0,0,“nb-iot.hu”,5684,0 //Open an SSL server connection
AT+QSSLSEND=0,0 //Send data to SSL server
AT+QSSLCLOSE=0,0 //Close the SSL connection

Currently, BC660K supports only PSK mode of DTLS