@random-username this is the setup i use in my house, the only difference is that i have setup static routes to apply qos even if someone connected to my DSL router wifi and not through DDwrt.
first plug the Ethernet wire to the WAN port of the 841n and setup your router exactly as in the pics
the internal IP is now on a different sub-net than the main
in this one only choose Gateway disregard the static routes.
if you didn't change anything in the networking tab you should have internet no problem at all.
@TheStunMan you are right public VPN and high download doesn't work, you still didn't enlighten me on how is an R8000 better for him than his 841n?
man i have a tl-wa830re and i flashed a wr841n v8 dd-wrt firmware on it and honestly even if i have a pppoe running with 2 V-APs and crazy routing and QOS the router handles it with a max of 20% cpu load, and ping time to router doesn't exceed 1ms