Solution for "lost signal" when using a third party router
I just rewired my home network to use a custom router (an Asus RT-AC88U) behind the AT&T modem/router combo, which created an issue with the U-verse IPTV. Upon switching channels, the TV would play normally for a few (10-20) seconds, and then freeze and display an error saying it lost the signal. I discovered the issue is due to multicast routing, and the solution for my router was to enable a setting for multicast routing in the Asus router. For my router, it's LAN -> IPTV -> Enable Multicast Routing. I also enabled fast leave and IGMP snooping, but those may not be necessary. My problem is solved and I'm just posting my solution here for posterity, so that it might help someone with the same problem in the future.