in the iptv section you don't need to specify an STB Lan port. If you do it stops that port working, as you've seen.
some notes I saved ages ago to get multicast streaming working..
In WAN -> PPPoE vlan id 101
In LAN -> IPTV,
- Don't set STB LAN port
- Leave Remove VLAN tag from DSL WAN unchecked
- Use DHCP routes - RFC3442 & Microsoft,
- Enable multicast routing (IGMP Proxy) - Enable,
- Enable efficient multicast forwarding (IGMP Snooping) - Enable
- UDP Proxy (Udpxy) - 0
In Wireless -> Professional
- Enable IGMP Snooping - Enable
- Multicast Rate(Mbps) - Auto
IPTV then works on any LAN port, and also over wireless if you have your Youview/BT UHD box connected to a wireless AP (I use a TP-LINK TL-WA860RE with my Youview box).