ISPs of any size will take more than one feed from their chosen ADSL/FTTC/FTTP suppliers for resiliency and load balancing (small operations may be resellers of a larger operation's service but Zen also has resellers and also wholesales it's backhaul but we digress)
This means that the gateway address changes each time the router makes a PPP connection attempt so that's why they cannot provide the information.
Can't speak specifically for Asus routers but had an occasion a few weeks back to switch my DNS servers from Zen (am using one of their fritzboxes) to Google as they had a DNS resolver issue but I had to be able to get online to work.
Although the router initially tied theirs it did switch to Google (I believe this is forced to facilitate their auto configuration of their Fritzbox suppled routers) but I did find it did take on board my change.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen => Freeola => Vivaciti (using O2 Wholesale DSL) => Xilo (C&W Wholesale) => Xilo (O2 Wholesale) => Xilo (TT Wholesale due to O2 Wholesale closure) => Zen LLU =>> ZeN FTTP (Openreach 300 Mbps down, 47 Mbps up)
Router: Fritzbox 7530
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.