@FTC
After further digging, I've found out the following.
I can either use AiProtection/QoS without IPTV or IPTV without AiProtection/QoS. Using any combination of both and whole thing just s**ts itself to death. I always had IPTV enabled because that is an absolute must, otherwise I have no purpose in even using router. Didn't expect something that's an absolute necessity to be causing problems.
Which explains why no one else is complaining about it. How many people even use AiProtection/QoS which are both disabled by default and further, how many of those use IPTV to route TV signal through the router to the TV box. It's annoying as F though since all this worked no problem on RT-AC87U. Must be something with chipset since this one uses different one and thus also different software parts. Merlin could probably fix it, but I don't know how much access he has to the closed source parts that connect to everything else, in this case AiProtection/QoS and IPTV. First are closed, IPTV is probably open.
Restored stock FW and reset everything several times and now I guess it's time to bitch at ASUS to look into it and fix it.