The logic here...
"If you get the Message 'Your STB is Blocked, Call Provider', then your Server URL is NOT blocked."
The service here sees your request but recognizes your device (browser) is not an approved device for streaming, so it rejects your request. Roughly translated: Your request reached the service here and had you been using a proper streaming device, you would have received the stream.
"If you get an Error 404 or similar message, you are being blocked by ISP for that URL."
Your request never reached the service here because something is blocking you. Roughly translated:. Even if you were using the proper device, you would not have received the stream because your request never reached the service here.