I think for ASUSWRT-Merlin that will work, his firmware is legendary, I have dropped ASUS DSL team an email to ask if the DSL-AC68U can be used behind another Router for DDNS ?
ASUSWRT-Merlin Changelog – 384.7 (7-Oct-2018) 
———–
– NEW: Added option to retrieve WAN IP from either the local
interface (like before) or through a remote server
(which works through double NAT) for DDNS.
———–
It is the WAN IP address that is the main issue I believe for the DSL-AC68U sitting behind another router .
In a completely sane world, madness is the only freedom (J.G.Ballard).
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