new-connection-mark=IPTV-Conn...add limit-at=15M max-limit=20M
While you can in principle get 20 Mbit/sec HD IPTV streams to work over WiFi, others downloading at the same time over the same shared medium — as in your original complaint — is not your only likely problem.
This looks like the classic sort of IPTV system where, if you're watching a 30fps video stream, each frame of video arrives at your LAN as close to 1/30th of a second apart as to not be worth quibbling over. This is the original definition of the word "streaming:" each bit of content arrives just in time to be played back, because the buffers in the sort of STBs used in those systems tend to be quite small by today's standards. They might buffer as much as half a second of video, but likely little more.
Contrast the likes of YouTube, NetFlix, etc., which may buffer 5-30 seconds of video before they begin playing it. Back when network speeds were closer to the size of the stream (e.g. 10 Mbit/sec with a 5 Mbit/sec HD video stream) you could see this buffering step occur before playback began. Now it happens in a fraction of a second in typical cases, but it still happens.
You may then ask, "Half a second is ~15 frames of video, so doesn't the IPTV system have that long to sort transient problems out before things start going bad?" Answer: no.
First off, classic IPTV streaming is a UDP-based protocol, meaning no retransmissions. A frame either gets to its destination intact, or it does not. If a frame spans 10 UDP packets, you only have to lose one to lose the whole frame, under many encodings.
Second, all "delivery" type video codecs for the past 3 decades work by making most frames refer to surrounding frames, to save bandwidth. "This frame is like that one over there, but with this section shifted right a bit, but also like that one over there…" This means that an error in delivery or decoding of one frame is likely to affect the 10-100 frames surrounding it, with typical encoding settings.
If your WiFi uses the 2.4 GHz spectrum, you can run into these sort of problems simply by someone nearby turning on the microwave oven, causing seconds worth of bad playback.
Or, it might work fine until you decide to have a house party, putting too many
ugly bags of mostly water between the sender and receiver, causing dropouts, since the very reason microwave ovens are effective is that water absorbs 2.4 GHz radiation.
My advice: stop trying to make WiFi do something it is ill-suited to. Run IPTV over wired connections, always and only.
Either that, or switch to over-the-top pseduo-streaming services like NetFlix, which are TCP-based and heavily buffered, which is why they work in the face of such problems.