Can't connect to a MQTT Server with supplied certificates

Hi there,
I am sorry if this is the wrong place to ask this, but I have been trying for the last weeks to connect to a MQTT Server for an application I am working on.

From this Server I got 3 files used as certificates:
- ca.pem
- mqtt.pem
- mqtt.key
Both pem files start with -----BEGIN CERTIFICATE-----
The .key file starts with -----BEGIN RSA PRIVATE KEY-----

My issue is that I can't seem to figure out how to connect to the Server with those certificates.

I did get an example from the Devs that made the Server, but it is in JavaScript:
```
var fs = require('fs');
const mqtt = require('mqtt')

console.log('mqtt test started...');

var KEY = fs.readFileSync('./cert/mqtt.key')
var CERT = fs.readFileSync('./cert/mqtt.pem')
var TRUSTED_CA_LIST = fs.readFileSync('./cert/ca.pem')

var PORT = 1883; // mqtt port
var HOST = '127.0.0.1'; // mqtt server

var options = {
port: PORT,
host: HOST,
key: KEY,
cert: CERT,
rejectUnauthorized: false,
ca: TRUSTED_CA_LIST,
protocol: 'mqtts'
}

console.log('try to connect....');

const client = mqtt.connect(options)
```

The example I got works perfectly, but since my project is in C# I can't use the one they provided.

Am I just getting stuck with a somewhat simple issue or is this something else?
I'd love to know how I should use those files in MQTTnet since everything I tried leads me to the error
`The remote certificate was rejected` and I even tried the mqttMultimeter, but I get the same error.
I even tried to convert the mqtt.pem and the mqtt.key to a .pfx file with openssl, but it still gets rejected.

### Which project is your question related to?
- Client

Start with the MQTTnet client certificate configuration and compare it with the supplied JavaScript example, including ca.pem, mqtt.pem, and mqtt.key. Reproduce the rejected-certificate error in the mqttMultimeter and C# client using the reported settings; done means the client connects successfully to the MQTT server with the supplied certificates.

Written by the indexing model from the issue text.