hmm not sure if the dir-615 has a bridge mode, so you might have to go with what you were thinking about disabling lan side dhcp etc. If the connection to your dir-655 is secure, like a physical ethernet connection then you can also disable the firewall, and maybe NAT though I dont see many options there so there might not be a "proper" way to do this. You would have to experiment for what works best for you, with stuff like DMZs and such. The dir-615 appears to be just a gateway, and not really designed (firmware wise) for pre-gateway routing. If you really need this then you would be looking at using dd-wrt or openwrt. Then you will be able to bridge wired or wireless. The worst case scenario if you just connect the dir-615 to your dir-655 and not change anything is having a double NAT that could affect voip.