Uploading large mail attachments with MS Graph API, running forever

I am trying to upload large attachments (>4 Mb). For that, the documentation suggests the following steps:

Creating a draft message -> starting upload session -> uploading attachment in chunks -> sending the mail

When executing the 3rd step, the code keeps running forever with no response (only started is printed, but not done). If I understand correctly, the requests.put() should give me the NextExpectedRange bytes to feed into the next put iteration. But as I said, it doesnt give me any response. I tried hardcoding the upload process of the chunks as you can see in the code.
In some sites, it was suggested that the chunk length should be dividable by 327680, thats why I have these 9 iterations. But even trying to upload the whole attachment as one chunk give me the same (no)reaction.

Messing with the header(e.g. taking out the Content-Length) gives me an InvalidContentRange error, changing put to post or the Content-Typegives me the same (no)reaction.

Needless to say, I have all required permissions.

Would appreciate any kind of help 🙂

#1. create draft
payload=json.dumps({       
"subject":subject,
"importance":"Low",
"body":{               
   "contentType":"Text",
   "content":content},
 "toRecipients":setup_recipients(recipients),
#"ccRecipients":setup_cc_recipients(ccrecipients)  
                })
 mail_data_draft = requests.post(url=ms_graph_endpoint_draft, data=payload, headers={'Authorization': 'Bearer ' + result['access_token'], 'Content-Type': 'application/json'})
 message_id=json.loads(mail_data_draft.text)["id"]
            
 #2. creating upload session with message id
            upload_session_endpoint="https://graph.microsoft.com/v1.0/users/" + email_account +"/messages/"+message_id+"/attachments/createUploadSession"
            up_session_payload= json.dumps({
                "AttachmentItem":{
                    "attachmentType":"file",
                    "name":attachmentName,
                    "size": contentSize                    
                }
            })
            upload_session_info= requests.post(url=upload_session_endpoint, data=up_session_payload, headers={'Authorization': 'Bearer ' + result['access_token'], 'Content-Type': 'application/json'})
            opaque_url =json.loads(upload_session_info.text)["uploadUrl"]
            next_expected_range=json.loads(upload_session_info.text)["nextExpectedRanges"][0]
            
            #####################################################################Tested till here 
            #3. uploading attachment

            #problem
            print("started")
           
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 0-327679/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 327680-655359/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 655360-983039/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 983040-1310719/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 1310720-1638399/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 16383400-1966079/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 1966080-2293759/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 2293760-2621439/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'327680','Content-Range':'bytes 2621440-2949119/3232962'})
            requests.put(url=opaque_url,headers={'Content-Type': 'application/octet-stream','Content-Length':'283842','Content-Range':'bytes 2949120-3232961/3232962'})

            print("done")
            # sending draft
            global ms_graph_endpoint
            ms_graph_endpoint="https://graph.microsoft.com/v1.0/users/" + email_account +  "/messages/"+message_id+"/send"