I doubt your *.pfx contains also the root certificate so it contains your signed certificate and the private key. Convert your *.pfx to *.cer (DER) or *.crt (Base-64) and import into STRUST. Do the same for your *.cer (DER) root certificate.
PEM certificates can't be mixed with other certificates since the whole trust chain including the private key can be contained in a PEM certificate.