@Joseph-TP All settings are at default. Your link talks about setting up IPTV via LAN. Everything works for me via LAN. It doesn't work via WiFi.

I have a TP Link Archer be 400. I used to have an Archer c6. I connected three TVs to the c6 via Wi-Fi, and everything worked fine. Now, after replacing the router with a be 400, the TV set-top box...