In the case of the WAN, that might be a different story. I checked my own ASUS RT-AC68U running the most recent dd-wrt firmware, and under PPPoE, there's an advanced settings option, and the following sub-options.
Now I can't tell if this is for the purposes of detecting tagged packets, or perhaps something intended for outbound tagging. But it's the closest option I could find.
If all else fails, you could get an inexpensive managed switch (sometimes referred to as a "smart" switch) to handle the tagging. Something that's at least 802.11q compliant (which the router is NOT!).
As a general rule, the router really isn't intended to support VLANs or VLAN tagging except under very limited circumstances. Given VLANs are proprietary, and the router supports literally 100's of different types of hardware, I believe it just wasn't practical to support them beyond a limited subset of Broadcom switches. But then again, I'm not an expert in dealing w/ VLANs on the router. That's simply the impression I've gotten having dealt w/ dd-wrt over many years.
That VLAN ID was for connecting to the internet through WAN. So something must be working inside dd-wrt, it is far more advanced than the stock firmware after all.