I have to agree with all what's said. ASUS has been the only DSL modem brand to cause problems on my line too. I've ran various other modem brands, that being the Huawei HG612, ECI /i, ECI /r, and now currently what appears to be stable and producing far less errors than the HG612 even - the Fritz!Box 7490 - now that I've got the public IPv4 subnet working without doing anything hacky.
I've put my ASUS DSL-AC68U back in the box and I will only get it out for purely routing purposes if I need a spare router. I don't trust the modem in it any longer unless the connection it's on is interleaved - which sucks of course.
This experience doesn't inspire me much confidence in buying products in future with the ASUS brand to be honest.