SSL connection not established due to certificate verify fail

Question

I have this Android app that is already published to the store, and was working just fine. Suddenly, it began to throw some exceptions, which I already explained and solved here. After solvind this issue, the app began to throw java.security.cert.CertPathValidatorException: Trust anchor for certification path not found. After some research, I found out that I had to create a HttpClientHandler to reference my certificate. So, here's my Login method, which is the first API call ever made in the app:

public static async Task<Usuario> Login(UsuarioLogin login)
{
    try
    {
        var handler = new HttpClientHandler();
        handler.ClientCertificateOptions = ClientCertificateOption.Manual;
        handler.SslProtocols = System.Security.Authentication.SslProtocols.Tls12;
        handler.ClientCertificates.Add(new X509Certificate2(Resources.certificadopem)); // certificadopem is my PEM format certificate file

        using (HttpClient client = new HttpClient(handler))
        {
            var json = JsonConvert.SerializeObject(login);
            var content = new StringContent(json, Encoding.UTF8, "application/json");
            var response = await client.PostAsync(ModelUrl, content); // exception is thrown when debugger tries to execute this line
            var response2 = await response.Content.ReadAsStringAsync();

            return JsonConvert.DeserializeObject<Usuario>(response2);
        }
    }
    catch (Exception e)
    {
        Console.WriteLine(e.Message);
        throw new Exception(e.Message);
    }
}

But now I am getting this exception: System.Net.Http.HttpRequestException: The SSL connection could not be established, see inner exception.

The inner exception is: {System.Security.Authentication.AuthenticationException} and the message says: Authentication failed, see inner exception.

The second inner exception is: Mono.Btls.MonoBtlsException and the message says: Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/boringssl/ssl/handshake_client.c:1132

I ran this openssl command openssl s_client -connect apigraos.copercana.com.br:443 to check the certificate authorities and this is the result:

C:\WINDOWS\system32>openssl s_client -connect apigraos.copercana.com.br:443
CONNECTED(000001B8)
depth=0 C = BR, ST = S\C3\A3o Paulo, L = Sert\C3\A3ozinho, O = COOPERATIVA DOS PLANTADORES DE CANA DO OESTE DO ESTADO SAO PAULO, CN = *.copercana.com.br
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 C = BR, ST = S\C3\A3o Paulo, L = Sert\C3\A3ozinho, O = COOPERATIVA DOS PLANTADORES DE CANA DO OESTE DO ESTADO SAO PAULO, CN = *.copercana.com.br
verify error:num=21:unable to verify the first certificate
verify return:1
depth=0 C = BR, ST = S\C3\A3o Paulo, L = Sert\C3\A3ozinho, O = COOPERATIVA DOS PLANTADORES DE CANA DO OESTE DO ESTADO SAO PAULO, CN = *.copercana.com.br
verify return:1
---
Certificate chain
 0 s:C = BR, ST = S\C3\A3o Paulo, L = Sert\C3\A3ozinho, O = COOPERATIVA DOS PLANTADORES DE CANA DO OESTE DO ESTADO SAO PAULO, CN = *.copercana.com.br
   i:C = US, O = DigiCert Inc, OU = www.digicert.com, CN = Thawte TLS RSA CA G1
   a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA256
   v:NotBefore: Apr  3 00:00:00 2023 GMT; NotAfter: Apr  5 23:59:59 2024 GMT
 1 s:C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert Global Root CA
   i:C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert Global Root CA
   a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA1
   v:NotBefore: Nov 10 00:00:00 2006 GMT; NotAfter: Nov 10 00:00:00 2031 GMT
 2 s:C = US, O = DigiCert Inc, OU = www.digicert.com, CN = Thawte RSA CA 2018
   i:C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert Global Root CA
   a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA256
   v:NotBefore: Nov  6 12:23:52 2017 GMT; NotAfter: Nov  6 12:23:52 2027 GMT
---

How can I fix this issue without ignoring all the security stuff?