If you can't make your primary router present both vlans on a single a trunk/tagged RJ45 port there's two options to pass them to your OpenWRT:
Option 1: Just use two cables.
Make on RJ45 port of your OpenWRT device for the first vlan and another one for the second.
Make one RJ45 port in your OpenWRT device "vlan 1 (LAN) untagged" and the other RJ45 port "vlan 99 (GUEST) untagged".
Make sure every RJ45 port has only one untagged vlan setting and every RJ45 port has all other vlans set to ("not member")
Option 2: Add a managed switch
Get yourself a managed switch.
Configured the switch just like I described the first option.
Make a third RJ45 port of your managed switch to "tagged vlan 1 and vlan 99".
Wire your managed switch to your OpenWRT device
Make the port on your OpenWRT device that connects to the managed switch as "tagged vlan 1 and vlan 99".
Better: Expose both vlans directly from your first router
If your primary router is e.g. OpenWRT as well, or any other router that is capable:
Configure a rj45 port on your primary router to "vlan 1 tagged, vlan 99 tagged"
Configure a rj45 port on your OpenWRT AP to "vlan 1 tagged, vlan 99 tagged"
And what now?
Now you have a "physical interface" called "br-lan.1" device and a "br-lan.99" device on your dumb AP.
Create two different "logical interface" (in the "interfaces" section of LuCI) and set up individual wifi SSIDs.