@Tarooo I found the solution: In the Deco app, go to VLAN/IPTV and instead of choosing your ISP, choose custom and activates VLAN only (with the entries that would be used had you chose your ISP).

@AdamJLoh Thanks for this post, it is driving me crazy trying to figure out the cause. I have the exact configuration. Can you forward the info on the special workaround?