The downloaded certificate is in cacert.der extension and Android 5.* does not recognise it as certificate file.
You can download the cacert file using your desktop machine and rename it from cacert.der to cacert.crt and drop it on Android device and certificate will be automatically added into file:///sd_card/downloads.
Installing the downloaded certificate.
Settings →Security →Install certificate from SD cards
Now, goto: sdcard →Downloads → Select cacert.crt
Now, Name it as anything “portswigger”
You also need to setup the PIN before adding certificate. Verifying the installed certificate using trusted certificates.
Trusted certificates →Users
After installing Certificate SSL endpoints also working fine tested using → https://google.com
After installing the certificate this way Firefox for Android won't use it (based on my tests), so use a different browser.