Hi GreenBay, thank you for your reply.
1. It does not work on any of my iOS devices, including iPhone 6s, iPad Pro and iPad mini. (iOS ver. 10.2.1)
2. I'm using a carrier in Hong Kong (1010). Even with wifi connections (both remote and local settings in mydlink app), the playback does not work.
3. I tried both Synology RT2600ac and netgear R7000, both cannot get it to work.
4. With the same router and carrier, android app is working perfectly. So by deduction, it has to be the iOS app problem.
Thank you in advance!
Best regards,
Nelson