So I've successfully installed LEDE on my Linksys WRT1900ACSv2 and setup all my wireless networks, SQM etc.
I now want to get my Netgear DM200-100EUS into modem/bridge mode and use my router to deal with the PPPoA settings. I am currently on an ADSL connection (BT, UK) and I have no idea how to do this. I'm not too familiar with ssh so it would be great if you could be really detailed in what I should do.
That is not going to work, PPPoA requires the device terminating the PPP-link to have a physical ATM-interface, but in your case only the DM200 has that interface. BUT as far as I can tell british links will mostly allow the user to switch between PPPoE and PPPoA, so on the WRT1900 you should be able to use PPPoE instead. Now PPPoE carries a bit more overhead than PPPoA but in the grand scheme of things this probably does not matter too much (a full MTU packet most likely will still occupy 32 ATM cells, so for full saturation loads you will net see an advantage of PPPoA versus PPPoE, for smaller than full MTU1500/1492 packets you might see a bit more throughput for PPPoA, but honestly that does not seem worth the hassle to me)
so can I use pppoe on lede and set the dm200 into bridge mode then?
From my, admittedly limited, understanding on the situation in the UK, yes that should work. Being over on the "continent" I can not test this hypothesis myself though, so I might be wrong. I am certain though that at least for some UK ISPs there is a automatic switch between PPPoA and PPPoE. Nut trying this should have no side effects except the link not working for the wrong method...