The way the DSL-AC88U was coded, It is a nightmare to figure out and best avoided if at all possible
The DSL-AC88U is a project not finished, but I would try Advanced Settings > LAN > Route – which can be used to connect several routers behind DSL-AC88U to share the same connection and DNS to the Internet
To manually change the DNS server of each node via the DHCP client (LAN) is not possible I believe on the DSL-AC88U
I can pass to you an unlocked DSL-AC88U firmware build which supports Telnet etc, but it is not a secure build 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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