[DSL-AC68U] WAN connected, clients can't connect to WAN

I've had a TP-Link DSL router operating for years, having used it as a replacement for the stock Verizon DSL modem/router. I decided to move to the DSL-AC68U for its built-in OpenVPN and greater configurability than the TP-Link. The TP works just fine, but has limited access to configuration items such as firewall rules, and no OpenVPN. The 68 model is getting on in years...but there doesn't seem to be an abundance of DSL solutions out there.

I found a new one on eBay for an OK price,...

[DSL-AC68U] WAN connected, clients can't connect to WAN