Question about ASUS RT-AX82U AX5400 WiFi 6 Gaming

I have connected the router via WAN input with a Lan cable to my Wingo modem. Everything is fine as far as the Internet and WiFi are concerned. But now when I connect the Wingo TV box to the Asus router, the TV doesn't work, I just have a black picture. When the TV box is connected directly to the Wingo Mondem, it works, so it must be the Asus router. It works when I change the mode to Acesspoint... but I don't want that... With my previous TP Link router everything worked in router mode. So I assume that it is some setting but I don't know which one and how to do it. Can someone help me? :-)