This could be due to the following reasons:
1. Your routers uPnP feature is Disabled.
2. Your router is not getting a proper public IP address from your Internet Service Provider. *This is common if your ISP modem is actually a Modem/router combo".
3. Your phone reception is weak.
4. Your phone provider has a firewall which is blocking the mydlink (Stunt/Relay) connection.
If you are using iOS (10.2 or higher) with T-Mobile, remote viewing and notifications may not work properly. D-Link is aware of the issue and is working on a solution.