openssl dgst -verify foo.pem expects that foo.pem contains the “raw” public key in PEM format. The raw format is an encoding of a SubjectPublicKeyInfo structure, which can be found within a certificate; but openssl dgst cannot process a complete certificate in one go.
You must first extract the public key from the certificate:
openssl x509 -pubkey -noout -in cert.pem > pubkey.pem
then use the key to verify the signature:
openssl dgst -verify pubkey.pem -signature sigfile datafile