Error when setting Root Certificate on SIM800L - ssl

Question

I desperately need some help on 2 questions:
I'm trying to connect a LILYGO TTGO T-Call SIM800L (IP5306 20190610) to AWS IoT, which requires an SSL authentication through a Root Certificate, a Client Certificate and a Private Key.
I'm able to successfully create the .crt files, write on them and set the Client Certificate through the AT Command:
modem.sendAT(GF("+SSLSETCERT=C:\User\clientcert.crt"));
But when I try to set the RootCA like this:
modem.sendAT(GF("+SSLSETROOT=C:\User\rootca.crt,1188"));
the GSM module returns "ERROR".
This is the documentation I'm using as a reference (Page 13):
https://microchip.ua/simcom/2G/Application%20Notes/SIM800%20Series_SSL_Application%20Note_V1.05.pdf
In the "Reference" section of the command "AT+SSLSETROOT", it's written "The files to be imported must be binary encoded". This confused me a little, so at first I simply wrote on the file the string format of the certificate, but I've also tried to change the extension to .der (which technically is the binary encoded format for certificates). I've tried to write on the file the hexdump version of the string format, and tried all the other avaiable extensions (.crt, .cer, .pem, .p12).
I've also tried to update the firmware following the procedure here: https://github.com/Xinyuan-LilyGO/LilyGo-T-Call-SIM800/blob/master/doc/How%20to%20update%20firmware.md
The only difference is that I used the Download Tool v1.10, because the other versions of the tool hanged on "Waiting" whenever I started the update. I retrieved it from here: https://simcom.ee/documents/?dir=SIM800x
I saw on other discussions that the SIM800L does not support TLS 1.2, but on page 6 of the SIM800 documentation regarding SSL, it's reported that "SIM800 series support SSL2.0, SSL3.0, TLS1.0 and TLS1.2."
On top of that, the command "AT+CIPSSL=1" works fine since it returns "OK".
I also read this: https://github.com/vshymanskyy/TinyGSM/issues/29#issuecomment-328802556
I'm attaching the function that sets up the certificates.
void setCertificates() {
modem.getModemInfo();
modem.sendAT(GF("+FSCREATE=C:\\User\\rootca.crt"));
modem.waitResponse();
modem.sendAT(GF("+FSCREATE=C:\\User\\clientcert.crt"));
modem.waitResponse();
char rootcertific[1188];
strcpy(rootcertific,rootCA);
modem.sendAT(GF("+FSWRITE=C:\\User\\rootca.crt,0,1188,1"));
modem.waitResponse(">");
SerialAT.print(rootcertific);
modem.waitResponse();
delay(1000 / portTICK_PERIOD_MS);
char clientcertific[2903];
strcpy(clientcertific,certificate_pem_crt);
modem.sendAT(GF("+FSWRITE=C:\\User\\clientcert.crt,0,2900,2"));
modem.waitResponse(">");
Serial1.print(strcat(clientcertific,private_pem_key));
modem.waitResponse();
delay(1000 / portTICK_PERIOD_MS);
modem.sendAT(GF("+FSREAD=C:\\User\\rootca.crt,0,1188,1"));
modem.waitResponse();
modem.waitResponse();
modem.sendAT(GF("+FSREAD=C:\\User\\clientcert.crt,0,2900,1"));
modem.waitResponse();
modem.waitResponse();
modem.sendAT(GF("+SSLSETROOT=C:\\User\\rootca.crt,1188"));
modem.waitResponse();
modem.sendAT(GF("+SSLSETCERT=C:\\User\\clientcert.crt"));
modem.waitResponse();
}
On page 11 of the SIM800 SSL documentation, the command AT+SSLSETCERT is described as "Client Client Certificate File with Private Key". Does this mean that I somehow have to write both the Client Cert and the Private Key on the same file?
As you might have noticed in the function, I used "strcat(clientcertific,private_pem_key)" and it returns no error, but I'm not sure whether this is the correct way to do this.
If anyone can help me out on this one I'd be infinitely grateful. It's been keeping me stuck for almost a month now.
Thank you!