@Virgo You're correct that the VLAN ID needs to be set, in CenturyLink's case to 201.  Two key things about the setup that ultimately enabled success.

#1 - Removed the CenturyLink router (what their "technical support" insists on calling a modem).  With the fiber optic service the connection to the "modem" is already an Ethernet WAN cable and no additional modulation / demodulation is needed.  This is better than putting the CenturyLink router into Transparent Bridging.

#2 - For the ER605, I anticipated using for Load Balancing.  For whatever reason, setting up the second port as a WAN when not yet connected prevented the first WAN port from working with CenturyLink.  Load Balancing that completely shuts down the TP-Link router when 1 WAN isn't present isn't helpful, and I'll need to look at that separately.

Thank you for your help and have a great day!