The tone-sets being cycled suggests to me that it's not configured appropriately for the Openreach network. How is the modem supposed to know which tone-sets it can and cannot use before or during the handshaking process? I'm not sure it can unless you configure it or the manufacturer configures it in the firmware for you.
Here is a comment in the AdslCoreDefs.h file from the Openreach HG612 source code release:
Revision 188.8.131.52 2010/01/07 03:49:11 l67944
The Huawei provided VDSL2 CPE (HG612) uses both A43c and V43 tone sets in its upstream handshake. Furthermore, upstream power back off is not applied in the V43 tones launched by the CPE. This means that the U/S handshake tones are violating the ANFP.
So Huawei had to adjust the HG612 firmware during development so that it did the right things.
The Lantiq dsl_cpe_pipe commands which include getting or setting the handshake tone-sets are llcg and llcs (low-level configuration get/set).
The ASUS modems have a setting in their GUI for enabling or disabling UPBO (Upstream Power Back-Off), if it were disabled, the upstream from a short line could adversely affect longer lines. And because people were having issues with it, they may have tried switching off and on all sorts of things, and most people can only observe how those settings affected their own connection.