IPTV feature require different VPI&VCI to your ADSL line. If you use different VPI&VCI, enable IPTV is correct.
If no, just use your current internet access and enable IGMP to access your IPTV

For MTU side, 1432 is at least ok for your internet, but i donot know if it is ok for your both IPTV and internet

Besides, a signle channel of 720P IPTV needs almost 4-6Mbps speed and might be more. However, a ADSL line have only a max of 24Mbps speed. General speaking, IPTV shares your internet speed both with internet access
What is your speed? Maybe it is that your speed is not enough who cause the IPTV slow