What you want is perfectly doable. However you'll have to reconfigure both devices (RB951G and hAP lite) for use of VLANs. Reconfiguration of both will be done in similar manner:
- you will use two VLANs: one for IPTV and one for LAN. Use any number between 2 and around 4000. Let's say you'll use VLAN ID 100 for LAN and VLAN ID 666 for IPTV
- let's say you'll use ether4 on both devices for IPTV (connection to router from RB951G and connection to set top box on hAP lite) and ether5 for connecting RB951G with hAP lite
- on both devices you'll configure ether4 as access port for VLAN 666 and ether5 as trunk (tagged) port for both VLAN 100 and 666.
- on RB951G you'll leave ether1 (internet connection) out of bridge and continue to use it as WAN port. The rest of ether ports (ether2 and ether3) and wireless interface will be configured as access port for VLAN 100
- likewise on hAP lite, ports ether1-ether3 and wireless interface will beconfigured as access ports for VLAN 100
You can configure VLANs either using bridge vlan-filtering or on switch chip on both devices. Bridge way is slightly easier to do but causes lower performance. Since you seem to lack knowledge and experience I suggest you to follow bridge way, which is nicely explained in
this tutorial. You can convert setup to switch chip later if you find performance drop unbearable.