Oh, well, right, my bad, I mentioned it's VDSL though.
Also, I think it's because of how frustrated I was about this fact, as I have a Broadcom, and ECI cannot set a target margin either, so stuck on 6dB.
Just to clarify, in case some people here aren't aware, although it's not a modem with a Broadcom chipset... the Lantiq/Infineon chipset offers the ability to change the downstream SNRM offset in 0.1 dB increments on VDSL2 and ADSL connections as well (where Broadcom chipset doesn't seem to do the same for VDSL2 connections). That is also assuming the connection isn't capped/banded and running at the maximum allowed speed.
Depending on what modem you're using which has a Lantiq/Infineon chipset, perhaps the ECI /r, something like the following should work:
dsl_cpe_pipe.sh locs 0 50
dsl_cpe_pipe.sh acs 2
This would effectively bump the downstream target SNRM up by +5 dB and cause the connection to re-sync with that change. The '50' value can be changed to another value between -50 to 50, in increments of 1 (effectively 0.1 dB). A setting of -30 for example would be -3 dB, so if you normally sync with a target SNRM of 9 dB then this would decrease it to around 6 dB for the downstream.
DrayTek routers usually offer this option as well, since they have a Lantiq/Infineon chipset, however it's done differently... such as:
vdsl snr -30
This setting should also be retained between reboots which is nice, if it's not then just do 'vdsl savecfg' to be absolutely sure.