Welcome to the RingCentral Community

I'm trying to get an access token for a new sandbox application using JWT, however the REST API keeps responding with OAU-153 Invalid Client ID. The client ID and client secret are definitely correct, as is my JWT token. Not sure what I'm missing here.

$RC_SERVER_URL="https://platform.devtest.ringcentral.com"
$RC_CLIENT_ID="T2V..." # Copied from the app I created
$RC_CLIENT_SECRET="v8dN..." # Copied from the app I created
$RC_JWT_TOKEN = "eyJraW..." # Created under My Account > Credentials 

# as per https://developers.ringcentral.com/guide/authentication/jwt-flow#technical-discussion
$splat = @{
    Method  = "POST"
    Uri     = $RC_SERVER_URL + "/restapi/oauth/token"
    
    headers = @{
        ContentType = 'application/x-www-form-urlencoded; charset=UTF-8'
        Accept = 'application/json'
        Authorization = 'Basic ' + [Convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes("${RC_CLIENT_ID}:${RC_CLIENT_SECRET}"))
    }
    body    = @{
            grant_type = 'urn:ietf:params:oauth:grant-type:jwt-bearer'
            assertion = $RC_JWT_TOKEN
    }
}

$Response = Invoke-RestMethod @splat

PS C:\scripts\Get-RingCentralData> $Response = Invoke-RestMethod @splat
Invoke-RestMethod : {
  "error" : "invalid_client",
  "errors" : [ {
    "errorCode" : "OAU-153",
    "message" : "Invalid client: T2VHgSXZQ92J0lu_htr0yQ",
    "parameters" : [ {
      "parameterName" : "client_id",
      "parameterValue" : "T2VHgSXZQ92J0lu_htr0yQ"
    } ]
  } ],
  "error_description" : "Invalid client: T2VHgSXZQ92J0lu_htr0yQ"
}
At line:1 char:13
+ $Response = Invoke-RestMethod @splat
+             ~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand

The one thing I find very confusing is when I go to Console > Apps > My New App > Credentials, the "User Credentials" section contains the dev environment's admin account with extension 101, and the info box says "use these credentials to test your app!"... well, I don't have access to that account, I want to use my own JWT to test the app. Is this what's causing me issues? Or is this just RingCentral trying to be "convenient" by showing me some credentials I could test with?

Any assistance would be greatly appreciated!