I'm having the same issue; I am (also was) using an R7000 and just switched to an ISP that requires a VLAN ID of 100 on the WAN port. I'm back online for the first time in almost a week having resorted to using a non-DD-WRT capable TP-LINK router with stock firmware (yuck!) that simply allowed me to enter the VLAN ID along with the PPPoE username/password, and voila! Back online! Except I don't want to use that piece of crap router; I want my DD-WRT back! It appears I'm going to have to confess to a loss of geek cred here, because I've tried everything to configure the R7000 running DD-WRT to work with my new ISP (yes, I've reviewed the guide) and no dice. Surely this router is not suddenly incompatible because of (what should be) a simple setting?
On a side note I also have a Linksys WRT3200ACM that I'd RATHER use, yet it has been sitting around collecting dust as I had issues with 2.4 GHz WiFi not working properly... separate topic for a separate thread, I'm sure, but figured it was worth mentioning.