I got a MAG 420 a year ago and ended up in the same situation so I had to use M3U. The issue is the portal is outdated and nobody bothers to update their portal with the patch to make it work with newer devices.
I've been using this service since February and it has been absolutely flawless and never had any buffering issues in the past, even watching FHD channels, no problem. However the past two weekends has been basically unwatchable, now it starts off fine for a minute but then I just get constant...