Actually playing with the settings:
1.
[PROVIDER IPTV] DVB Inputs -> Networks -> Maximum # input streams: 1
Seem to work fine. When I am streaming in one client I am unable to connect with second client. Is it that easy?
3. For test I added a second IPTV provider and set a lower priority to it.
In DVB Inputs / Muxes I picked one channel [from PROVIDER IPTV] and set high priority to it (priority & streaming priority -I don't know which one helped).
Also I selected a failover-channel [from SECOND IPTV PROVIDER] here and set lower priority to it (priority & streaming priority)
In Channel/EPG -> Channels I chosed the channel I selected earlier [from PROVIDER IPTV] and added failover-channel [from SECOND IPTV PROVIDER]
It seem to be working. When I try to connect to it I see main channel [from PROVIDER IPTV], and failover-channel only then, when [PROVIDER IPTV] is busy (streaming).
A new question has arisen...
* how do I batch add a failover service/channel/ for all channels? Setting one by one is very time consuming
* what would be the best way to stream a still image and use as IPTV source? I tried ffmpeg -re -loop 1 -i ./image.jpg -r 1 -vcodec mpeg4 -f mpegts udp://0.0.0.0:3322, but it uses a lot of cpu.