API call from https node application never reaches the destination - node.js

Question

I have a node.js application served over https. I would like to call an API from that application. The API is also served over https and it has been generated using the express-generator.
Unfortunately the call never works. There is no error message. The call never reaches the API.
Strangely enough if I try to call another public API (e.g. https://api.publicapis.org/entries') that is working perfectly.
Here is my call:
const requestBody = {
'querystring': searchQuery,
};
const options = {
rejectUnauthorized: false,
keepAlive: false, // switch to true if you're making a lot of calls from this client
};
return new Promise(function (resolve, reject) {
const sslConfiguredAgent = new https.Agent(options);
const requestOptions = {
method: 'POST',
body: JSON.stringify(requestBody),
agent: sslConfiguredAgent,
redirect: 'follow',
};
fetch('https://192.168.112.34:3003/search', requestOptions)
.then(response => response.text())
.then(result => resolve(result))
.catch(error => console.log('error', error));
});
};
And here is the API which I would like to call:
router.post('/', cors(), async function(req, res, next) {
req.body;
queryString = req.body.querystring;
let data = JSON.stringify({
"query": {
"match": {
"phonetic": {
"query": queryString,
"fuzziness": "AUTO",
"operator": "and"
}
}
}
});
const { body } = await client.search({
index: 'phoneticindex',
body: data
});
res.send(body.hits.hits)
});
What is wrong with my API and/or the way I am trying to communicate with it?
UPDATE: I receive the following error in the fetch catch block: 'TypeError: Failed to fetch'
When I create a request in Postman I receive the expected response.
UPDATE 2: This is most probably an SSL related issue. The webapp is expecting an API with a valid certificate. Obviously my API can only have a self signed cert which is not enough here. How can I generate a valid cert for an API which is running on the local network and not publicly available?
UPDATE 3: I managed to make it work by changing the fetch parameters like this:
fetch(url, {
method: 'POST',
headers: {'Content-Type': 'application/json'},
mode: 'cors',
body: raw,
agent: httpsAgent,
redirect: 'follow',
})
and on the API side I added the following headers:
'Content-Type': 'application/json',
'Access-Control-Allow-Origin' : 'https://localhost:2200',
'Access-Control-Allow-Methods' : 'POST',
'Access-Control-Allow-Headers' : 'Content-Type, Authorization'
I also added app.use(cors()) and regenerated the self-signed certificates.