Continuing the discussion from TP-Link TD-W8970B internet not working (LEDE v17.01.1):
Hopefully this helps someone. I've spent ages trying to sort out my WAN settings. My ISP is TalkTalk, using their FTTC service, and I'm using LEDE 17.01.04 on a BT HomeHub 5a with inbuilt VDSL modem by Lantiq (though this is more about the connection settings than LEDE itself). After I installed LEDE I trawled the forums (TalkTalk and elsewhere) to find the right connection settings. They all talked about VDSL using PPPoE. It was fairly easy to get the DSL connection up but a complete pig to get a PPPoE connection to establish. Once it was up it was fine but it could take anywhere from 10 minutes to hours and hours to establish a connection: the logs showed IPCP was providing my router with IP addresses but the session would then hang up.
Anyway, long story short, eventually I found the thread linked at the top and noticed the Irish ISP router settings page which looks very similar to the TalkTalk-provided router settings. I followed the advice in that thread and it turns out TalkTalk uses DHCP. I thought that was only used in FTTH connections but no, apparently TalkTalk uses DHCP for FTTC connections as well. So once you've got your VDSL link up, which is quite easy (use VLAN 101 so you'll be using ptm0.101 not ptm0) you just need the following in /etc/config/network :
config interface 'wan'
option proto 'dhcp'
option ifname 'ptm0.101'
The connection now comes straight up within seconds and is rock solid, with a 15 minute DHCP lease time. I'm a little puzzled how I ever got a connection up with PPPoE in the first place, but however that happened it's much easier and much more reliable using DHCP. Hope this helps someone.