use STB router as Switch

Hello!! I have a router (Huawei EchoLife 520) with STB port for IPtv. I want to connect this router to my Prime router (Linksys) to act like switch so i can connect it to my Set top box (via STB port) because linksys has not have STB port for IPtv The reason is that linksys sync @ 20Mbit and Huawei as Prime router syncs @ 8mbit!!!! Ive try to do it (diable dhcp,nat firewall), and Huawei act like switch. Ethernet ports 1 to 3 shares internet to other devices but from STB port my Set top box returns DHCP error. any help??

yes i have done that from the beginning and i can share internet throu the Ecolife but not IPtv.
After so many trial and error i think theres no way to have iptv if the "master" router hasnt have multiple vc's.