Maybe an option (for testing only) to try @Grisu suggestion of GNUton firmware on your DSL-AC68U as it is built in part using ASUSWRT-Merlin firmware. hopefully GNUtons build has included the DDNS option for Double NAT
Then this should be a good indication if this option will support DDNS and a good reference to others also in this dilemma.
I have not tried DDNS behind double NAT, maybe others on this forum have ?
In a completely sane world, madness is the only freedom (J.G.Ballard).