Forcing G.DMT Modulation on a DGN3500

ADSL2/2+ doesn't work very well for lines with high attenuation as it spreads the downstream channel across higher frequencies than those used by ADSL1.
Higher frequencies are more sensitive to attenuation than lower frequencies causing a drop in noise margin. You may see a slower sync rate or even disconnects if it's severe.

Most ISP's default to ADSL2/2+ modulation. Sky made a move to ADSL2/2+ a year or two ago and I saw a drop from 6000kbps to 3000kbps on the downstream due to it.
Luckily due to the efforts of individuals and online communities, it's possible to force the DG834GT to use ADSL1 modulation bringing back the speeds lost when using ADSL2/2+.
I bought the DGN3500 to replace the DG834GT as I needed a gigabit ethernet router & modem. After browsing the webui, telnet, GPL sources, and various forums; I concluded the DGN3500 could not have its modulation forced. Few weeks ago I decided to backup my settings and perform a factory reset on the router as a recent update botched my port forwarding settings. I noticed something in the configuration file:

[30004]"DSL Mode"=MMODE

MMODE meaning multimode. So I re-configured my router, backed up my settings to netgear.cfg, and edited that line to

[30004]”DSL Mode”=GDMT

To force ADSL1 modulation.
And it works :)