When you are inside your LAN, i.e. hardwired or on wifi, you use the internal IP address of the DiskStation to access it. The router is out of the picture.
When you are outside your LAN, including on a smartphone/iPad with wifi turned OFF and using only 3G/4G, you use your ISP's assigned IP address, or better yet, a fully qualified domain name assigned to it.
DS 1512+ w/3GB, 5 x 3TB Seagate ST3000DM001 8.2TB RAID 6, half files/half Time Machine.
Icy Dock MB559U3S-1SB enclosure w/4TB Hitachi UltraStar via USB 3 for files backup
UVERSE to AirPort Extreme + 2 AirPort Express w/speakers. TRENDnet TV-IP312WN camera
CyberPower CP1500PFCLCD Sine Wave UPS
DSM 4.1-2661 w/SSH + SFTP, VPN Server, Syslog Server, Media Server, Mail Server, Mail Station,
Audio Station, Surveillance Station, Photo Station, Web Station - DS Apps on iPad & iPod Touch.