There is a little progress however this makes the whole issue even more weird. Changed the ASUS router settings from Access Point to Router (disabled DHCP so KPN Experia box is responsible for this) and the TV is still freezing but only when I change the channel. After a minute everything works perfectly.

Let me answer my own question as nobody from KPN (including Tech Desk) was useless at the end. But at least you can learn how to setup an Access point.

Spent 1.5 hours talking with a guy from Tech Desk who was very nice and helpful but useless. After 15 minutes he screwed up my settings so much that he could not make my TV work. I had to jump in and fix everything. Then we agreed somebody will call me back later to help me. So far nobody called me… So cancelling the Tech Desk subscription as it was a total waste of 5 EUR and 1.5 hours.

I talked with ASUS Support and it turned out they don’t want to add KPN as a preset to their routers and they are not supporting IGMP Scooping together with configured VLANs. Shame for them because it makes their products not working together with KPN Expria Box.

But there is a solution!!!

Cheap unmanaged switch! I bought a TP-Link TL-SG1005D switch and installed it between the Experia Box and the IPTV box. Now the TV works properly and I could connect the ASUS Router/Access Point to the switch too. Now everything works properly.