Iit should be noted that the performance of the device web interface is reduced due to the length of the key.
Certificate installation
Creating a self-signed certificate with OpenSSL
Self-signed certificates can be generated with a script:
#!/bin/sh
CERT_PATH="."
CERT_NAME=${CERT_PATH}/"custom"
CERT_SUBJ="/C=SK/ST=Bratislava/L=Bratislava/O=Vutlan s.r.o./OU=IT Department/CN=vutlan.com"
openssl req -new -newkey rsa:2048 -days 365 -nodes -x509 -keyout "${CERT_NAME}.key" -out "${CERT_NAME}.pem" -subj "${CERT_SUBJ}"
Then install files custom.pem and custom.key in accordance with the certificate installation procedure.
To install your own certificate need to copy certificate file (s) on the USB-flash stick and rename them respectively custom.pem and custom.key. Then connect the USB-stick to the unit. After the completion of the exchange (the LED on the USB-stick is flashing on and off), remove the USB-flash drive and open the web interface via HTTPS.
Or upload certificate and key file by Network settings.
Do not forget to enable HTTPS mode as written here: Network settings.
Certificate installation for versions 
2.5
Creating self-signed certificates by OpenSSL
Self-signed certificates can be generated by:
openssl req -x509 -nodes -days 365 -newkey cert.key -out cert.crt
Then combine with text editor both files cert.key and cert.crt in next steps: in the beginning content of cert.key, then the content of cert.crt. Save file like custom.pem, copy it to the USB-flash and put USB stick to device USB port.
To install your own certificate file copy it to the USB-flash, rename file to custom.pem and put USB-flash to device USB port.
After the completion of the exchange (the LED on the USB-flash drive and flash off), remove the USB-flash and open the web interface over HTTPS.