I'm pretty sure SNRM adjustments on devices Broadcom chipsets will only work on ADSL, not VDSL2, unlike on the DrayTek where it actually works on VDSL2 as an offset. If I'm right then you just need to cap the sync rate instead (which does work on devices with Broadcom chipsets). It's something like 'xdslcmd configure --maxDataRate [downstreamKbps] [upstreamKbps] 100000', where downstreamKbps and upstreamKbps are values of your choice.