i am not with plusnet but under normal set up should be like this

WAN >> General Setup > wan 2 (disable wan 1)
enable yes
Display Name may need give name (fttc or plusnet)
VLAN Tag insertion may NEED enable and vlan tag value i think plusnet use 101
active mode always on

WAN >> Internet Access > wan 2
PPPoE top left enable
username & pass may need to use the online account or whatever plusnet use
mtu 1492

EDIT

you may also need ... System Maintenance >> TR-069 Setting > acs server (internet or wan 2)
enable the ACS server and wait 15 min if will update the connection

I have my 2830N talking to Plusnet via PPPoE.
Disable WAN1 Enable WAN2 as Always On
Enter your username and password in the details page.

PPP Authentication is PAP or CHAP

That should do it.