You simply need to select PPPoE and enter the username and password from your Zen portal.
I've been using the Asus RT-AX86U for the past 15 months and think it's an excellent router.
I run the 3rd party Merlin firmware on it for a few extra features and bug fixes.
It depends who Zen gets your FTTP from. If it's CityFibre, like mine, it needs the VID, 911, setting - Advanced Settings/LAN/IPTV/Internet