Depends on how
exactly your ISP delivers services. But let's assume its like this: you get PPPoE untagged and IPTV tagged vith VLAN ID 1000. Both services are passed over same physical connection.
Now you have to create something that will pass VLAN ID 1000 to port where you have IPTV clients. Ideally you would configure VLANs according to
this (general) VLAN guide. WAN port will be hybrid port: untagged for PPPoE and tagged for IPTV. Then you would add another port to VLAN 1000 ... what kind very much depends on how IPTV client expects it. E.g. if it expects IPTV service over tagged, then this port will be tagged member of VLAN 1000. You will probably want to allow also some internet access to IPTV client, possibly over untagged, so the port on RB in question will at the same time be untagged for appropriate LAN subnet (I'm using separate subnet for IPTV boxes but you could add those to normal LAN if you trust your ISP).