Calling API returning Encoded Characters - encoding

Question

We are calling an API which is external to us and we have no control over. When the data is returned characters are encoded with �. We are using fetch in javascript like so:
return fetch(apiUrl,
{
method: "GET",
headers: {
'Accept': 'application/json, charset=UTF-8',
"Content-Type": "application/json; charset=UTF-8;",
'Accept-Charset' : 'utf-8'
}
})
.then((response) => response);
The first thing we do with the response is call response.json(). At this point the � has replaced any currency symbols or accented letters. I have tried immediately calling response.text() to get the raw text with the same result. Also when I view the result in Fiddler I am seeing � instead of currency symbols.
I have added UTF-8 in the headers to try and accept the right data but with no luck, is this an unsolvable issue down to the external API (internal to company but cannot be changed or exposed here) or am I missing something obvious?
UPDATE:
Following the suggestion below I have removed charset and re-run and interestingly if I view the response in fiddler on the HexView tab it is displaying correctly (although it says "Response body is encoded. Click to decode"), once I click that it displays correctly in HexView. It isn't in the TextView, Raw or JSON tab. The beginning of the HexView data shows:
HTTP/1.1 200 OK
Cache-Control: private
Content-Type: text/plain
Vary: Accept-Encoding
Server: Microsoft-IIS/8.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
X-UA-Compatible: IE=edge
Date: Thu, 26 Jul 2018 07:52:40 GMT
Content-Length: 14876