Why generated signature by "Digital signature algorithm (SHA-256)" with CMS is not valid in my partner

Question

I have big problem with signature I don't recognized why it's not work.

I want to use AS2 over http with digital signature.

Minimal Story:

I generated private key, certificate and as result .pfx file by following commands:

openssl genrsa -out key.pem 2048

openssl req -new -sha256 -key key.pem -out csr.csr

openssl x509 -req -in csr.csr -signkey key.pem -out cert.crt

openssl pkcs12 -export -in cert.crt -inkey key.pem -out certificate.p12

And firstly I just tried

1. hash my "data" by sha256
2. sign data with rsa.

When I had sent it, I encountered an error. Error that not recognized hash algorithm.

Question: this error mean that I must use "RFC 6211 - Cryptographic Message Syntax (CMS)"?

And next I tried this c# code:

public byte[] SignData(byte[] data, string p12FilePath, string p12Password)
 {
     X509Certificate2 signingCert = new X509Certificate2(p12FilePath, p12Password);

     ContentInfo content = new ContentInfo(data);

     SignedCms signedMessage = new SignedCms(content, false);

     CmsSigner signer = new CmsSigner(SubjectIdentifierType.IssuerAndSerialNumber, signingCert);

     signer.DigestAlgorithm = new Oid(Oid.FromFriendlyName("SHA256", OidGroup.HashAlgorithm));
     signer.IncludeOption = X509IncludeOption.WholeChain;

     signedMessage.ComputeSignature(signer);

     byte[] signedBytes = signedMessage.Encode();

     return signedBytes;
 }

After that I load data to my http and as result it's have this http format:

 // Create multipart form data
 string boundary = "STARTBOUND_" + Guid.NewGuid().ToString() + "_ENDBOUND";

  var formData = new MultipartFormDataContent(boundary)
  {
      Headers =
      {
          ContentType = new MediaTypeHeaderValue("multipart/signed")
          {
              Parameters =
              {
                  new NameValueHeaderValue("boundary", boundary),
                  new NameValueHeaderValue("protocol", "\"application/pkcs7-signature\""),
                  new NameValueHeaderValue("micalg", "sha1")
              }
          },
      }
  };


 var fileContentPart = new ByteArrayContent(as2Message);
 fileContentPart.Headers.ContentType = new MediaTypeHeaderValue("application/EDIFACT");
 fileContentPart.Headers.Add("Content-Transfer-Encoding", "binary");
 fileContentPart.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
 {
     FileName = "data.edifact"
 };
 formData.Add(fileContentPart);



 var signatureData = SignData(as2Message, _PathToMyPrivateKey, _Passphrase);



 var signatureContentPart = new StringContent(Convert.ToBase64String(signatureData));

 signatureContentPart.Headers.ContentType = new MediaTypeHeaderValue("application/pkcs7-signature");
 signatureContentPart.Headers.ContentType.Parameters.Add(new NameValueHeaderValue("name", "smime.p7s"));
 signatureContentPart.Headers.ContentType.Parameters.Add(new NameValueHeaderValue("smime-type", "signed-data"));

 signatureContentPart.Headers.Add("Content-Transfer-Encoding", "base64");

 signatureContentPart.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
 {
     FileName = "smime.p7s"
 };
 formData.Add(signatureContentPart);

When I had sent it I received another error: Outbound MDN details: Error verifying the senders digital signature: Verification failed

I 1000 times checked my certificates and all is correct. My partner tried add this certificate (which I sended to him) to Trusted Certificates store, but it's not helped us. But I don't understand why this error.

In internet I searched that exist

1. Signature
2. CMS with Sinature (Please explain me one thing: Can I create CMS from scratch? and where example of it. Because in rfc I have only this

" SignerInfo ::= SEQUENCE {
           version CMSVersion,
           sid SignerIdentifier,
           digestAlgorithm DigestAlgorithmIdentifier,
           signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
           signatureAlgorithm SignatureAlgorithmIdentifier,
           signature SignatureValue,
           unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }"

)

My partner has "Use "Algorithm Identifier Protection Attribute" in signature "in Mendelson and SHA-256 algo to digital signature algorithm What it's mean?

Please give me some help)