Using TLS with SIM7080 (SIM7000) AT commands. How to deal with certificates
Solution 1:
You must upload your certificate to the module first (one time) before calling AT+CSSLCFG="convert",2,"mycert.crt"
At least the .pem format works.
AT+CMEE=2 gives verbose errors, but I am not sure if you get more information on this error
The header and body length are max lengths, so those are not the cause of your issue.