Well, I'm not sure what your best option is but maybe at least understanding what is needed and why will help you find things to try. The big issue is that your ISP doesn't just send you all the packets for all the IPTV channels simultaneously, and so there needs to be some communication between your TV device and the ISP to request the packets needed for the channel of interest. This communication occurs via IGMP packets which are not IP packets and do NOT route. So some software has to get them onto the ISPs wire so the ISP multicast routers will hear them and will do their thing and start sending you TV packets.
smcroute can route specific fixed multicast streams, and might not have the pppoe bug, but it probably won't work for you because most likely you have many channels and each one is a different multicast destination.
The next thing you might be able to do is run some sort of multicast routing PIM type daemon, but I doubt that the ISP will listen to your PIM, they are probably only listening for IGMP. To do so you'd have to know the IP address of your own ISP's PIM routers, and you'd have to have them be willing to listen to your PIM packets which arrive via regular unicast IP. I doubt that is likely to happen.