Since you flashed Merlin you could try the DSL Merlin version for the DSL-AC68U by gnuton:
https://github.com/gnuton/asuswrt-merlin.ng/releases
Which is Merlin with DSL support.
I recommend latest STABLE release:
https://github.com/gnuton/asuswrt-merlin.ng/releases/tag/384.12-gnuton1
But most likely it doesn't affect the DSL firmware, it's smaller size like the restoration firmwares, of which I believe don't hold the DSL firmware which you need... but who knows, doesn't hurt to try.
Mine has same issue, now upgrading the 3rd time the "fix" version, so far no firmware is mentioned under "DSL log".
Though pretty sure if you want to flash the DSL part of the firmware you need to use the GUI=webbrowser upgrade, since the restoration versions are smaller, don't mention dsl in the filename, so pretty sure it does not flash the dsl portion, that's why you need to flash the full firmware afterwards through the GUI=webbrowser.
For me the restoration tool always cancels around 40%... Think my router is bricked...
Never flashed custom firmware. Simply stopped working someday...
I live in Thailand, tropical climate, it's around 4 years old... Think got overheated, I hear rattling of plastic parts inside, pretty sure the plastic around the screws hardened and cracked / chipped off... Maybe a hardware failure...
Very dissappointed since I paid over 200 EUR for it, and the old simple routers of 10 years ago would last forever... guess that's the downside of high end hardware these days: higher spec CPU generating more heat, life span much shorter...