A RT-AC86U (lovely device) The Zyxel VM1312 contains the BCM63168U chipset (Broadcom) the DSL-AC68U uses a MediaTek chipset for DSL which for UK users has caused many issues and G.inp is not usually supported.
So use Zyxel as Modem (Bridge) then uses RT-AC86U (as primary AiMesh) and set DSL-AC68U as Child/Node (AiMesh) the latest DSL-AC68U firmware supports AiMesh for child /node option.
Version 3.0.0.4.384_81140 (from ASUS download Servers)
I was using AiMesh and it worked well ;-)
BTW – on the rear of the RT-AC86U, there is a label with a year of manufacture on it, I cannot recall but I seem to remember units made in 2017 may have had (some) 2.4GHz wifi radio issues and others have reported that on occasions the router will not restart after it has been UI rebooted ?
No such thing as the perfect router, but that’s the price for mass production 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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