Azure Function App error " remote name could not be resolved"

Question

I have a azure function app which is reading a file. It returns the following error:

remote name could not be resolved

Code is just this like:

byte[] attachmentContent = new System.Net.WebClient().DownloadData("https://www.example.com/Document?Seqn=588736&Ikey=DF466F2A-FD14-4C48-A3E2-5E692E2EF4C9&Aseqn=1764453&noleft=1");

Edit:
1. I can open url in browser and it starts downloading it.
2. Running locally it works fine.

Help please