I own a Zyxel VES-1608FA-35 DSLAM (why not) but it's quite old and not 100% compliant to standard.
In every place I know where Ikanos DSLAMS where deployed (Belgacom - Belgium and Bell Stinger - Canada) usually only Ikanos Modems or specially modified ones (special mode which can be enabled in Firmware) are reported to work at all. But all of this modems are getting quite old now (and I would expect the DSLAM to outlive the el cheapo CPEs...)
TLDR; Now to the question:
How much of the Handshake/Training protocol is handled on the CPU (kernel module and user space daemon) vs the firmware blob?
Best case just some pre-standard feature flags need to be send/accepted and the lantiq can sync - Worst case a SDR firmware patch would be needed which would be a dead end of course (unless there is Ghidra support for the architecture and the blobs aren't signed, but still meh)
The more magic is happening on the CPU (and the code to which we have sources), the higher are my chances to debug/understand the issue.