You do not necessarily need to tag the packets on the OpenWRT router (or use a VLAN at all). If you only want it to provide a guest network, you can set it up like any other AP and connect it to one of the LAN ports of your EdgerouterX and configure a separate network for that port on your Edgerouter (as far as I know, the EdgerouterX really has multiple physical interfaces as opposed to your AP, which most likely only has one and an integrated switch). To do so, you can follow this guide, using e.g. "eth2" instead of "eth1 vif 10". Optionally, you can modify the firewall configuration in the OpenWRT AP so that it simply bridges the WiFi without routing it, as the Edgerouter already does NAT for you (saves you from double-NAT).
If you want to use VLANs in OpenWRT, you first need to configure the VLAN on the integrated switch of your OpenWRT device (LuCI: Network->Switch, add) and select "tagged" on the port you connect to the edge router and on the "CPU" port. After this, it is easier to just create a new bridge (Network->Interfaces, Add new), covering "VLAN Interface: eth0.X" and then changing the wireless configuration to bridge to it instead of to the LAN bridge (Network->Wireless->Edit). Since you do not need to route, you do not need to configure firewall rules for the new bridge as everything is handled on the EdgeRouter. The advantage this setup has is that you separate the traffic from OpenWRT into management (untagged) and client traffic (tagged).