You will need 3 separate entries, one for each port forwarding. See my example below. The Private IP Address is the IP Address of the Tablo. However, your router likely assigns the Tablo’s IP via a DHCP server where if the router is restarted another different IP address will be assigned. You can imagine if the Tablo’s IP changes this will break the port forwarding. So you need to assign a static local IP to the Tablo. You do not need a Static WAN IP (aka a static IP from your ISP), this can be dynamic.
Inbound port 21012 to 21012
Format TCP
Private IP Address 192.168.x.xxx
Local port 8887 to 8887
For how to set up DHCP Reservation (aka a static local IP) for the Tablo, go to page 18 in the link below. Section 3.5.1 in the PDF lets you create a “fixed DHCP lease”, that’s what the manual calls DHCP Reservation.
http://h30434.www3.hp.com/psg/attachments/psg/Networking/39032/1/ARRIS_Router_Setup_Web_GUI_UG.pdf