Router as switch, IPTV not working properly

Hello guys,

I’m trying to repurpose an old Archer C7 v2 with OpenWRT to work as a wired switch/wireless AP on my current home network. The main router is a Huawei HG8247G with custom Vodafone firmware which I can’t mess with much.
I followed info on wiki to set up OpenWRT through LuCi to work as a secondary router (DCHP disabled, WiFi set with same SSID as main). I also activated IGMP snooping as I read that my provider’s IPTV box requires IGMP snooping v3 to work. I edited config files to force v3 and also multicast_querier, as I found on the forum from people with similar problems.

The current result: I can access the router from within the network, WiFi seems to be working, and I have my IPTV box working... for one minute. Then it freezes image/audio, gets me a connection error, and after another minute, I have signal, for another minute. And so on, and so on.

I have some -very- basic Linux/networking notions, but I’d like to make this work. However, I’m feeling a little lost here. Can someone point me where should I start to troubleshoot this?

Best regards and thanks in advance!