AFP/IPTV Port Forwarding Not Working

Hello all!

This is about a Time Capsule 4th Gen that I'm trying to set up as a remote file server/wireless AP in conjunction with the telecom provided router.

Why I want to use two routers? The one provided by the telecom company is simply trash: it does not provide adequate coverage for one room, only does 100Mbps LAN (internet connection is probably bottlenecked by this as I get 90Mbps), has flashing lights in the front that are quite annoying and a couple more reasons, but mainly the first two.
The catch here is that the telecom router has IGMP snooping, which allows it to tap the IPTV broadcast signals, and this is absolutely necessary for my TV to work.

The challenge is to get either the Time Capsule as the main router with the telecom router as a bridge providing IGMP snooping for IPTV, or to have the Time Capsule in bridged mode and forward port 548 to the Time Capsule so remote access is possible.

So far I've failed to redirect IGMP requests from the telecom router (both in double NAT and bridged scenarios) through the Time Capsule. I don't think it's as simple as plugging in the port numbers and expecting it to work, as that clearly didn't.

The second scenario I've had more success with, with the Time Capsule being easy as pie to set up in bridged mode. But the friggin' port forwarding does not work on the telecom router. There are multiple functions under its "Firewall" menu, including Port Forwarding, DMZ and Packet Filtering. I'm not sure what I'm doing wrong here but simply forwarding port 548 to the Time Capsule just doesn't work...

Does anyone have suggestions for getting everything working? Did I miss a port or is this a common issue?