Use X_Forwarded for

@app.route("/printers/announce/<name>/<key>")

def announce_printer(name, key):    

    cfg = json.dumps({"hostname": request.remote_addr, "port": 5000, "key": key})

    ip = request.environ['REMOTE_ADDR'] if request.environ.get('HTTP_X_FORWARDED_FOR') is None else request.environ['HTTP_X_FORWARDED_FOR']

    cfg = json.dumps({"hostname": ip, "port": 5000, "key": key})

    log.info(f'Registring printer {name} with config {cfg}')

    printer = get_printer_by_name(name)

    if printer == None: