Ultimately this is more of a TP-Link question than a pfSense question, but...
@erichium
Can I set up VLAN support in pfSense such that the wifi traffic from the TP-Link router goes through one VLAN and the hard-wired traffic from the TP-Link router goes through another VLAN? I've seen that the router has support for IPTV/VLAN, but I think I'm coming to the conclusion that, that's not really supporting VLAN for pfSense.
IPTV/VLAN should solve this for you.
On the AX1800, the IPTV/VLAN feature lets you configure up to 3 VLAN tags on the AX1800's WAN port. You can then choose which LAN ports are on which VLAN.
Limitations
You can't name the VLANs. On the tp-link the VLANs are named "Internet", "IPTV", and "VOIP", but they can be for whatever you want.
Only the WAN port on the tp-link supports vlan tags. LAN ports have untagged traffic, but you can choose which network each LAN port is connect to.
All WiFi traffic goes to the "Internet" vlan on the WAN port, regardless of SSID.
Steps on the AX1800
Advanced -> Network -> IPTV/VLAN
Enable "IPTV/Vlan"
Mode: Custom
Enable "802.1Q Tag for internet". Set the VLAN ID to some number (ex: 2)
Enable "VOIP VLAN". Set the VLAN ID to some number (ex: 3)
Disable "IPTV VLAN".
Set all 4 LAN ports to VoIP. The WiFi is always on the "Internet"
Save.
Connecting things
Connect WAN port of the tp-link to one of the LAN ports on the pf2100. Configure pfsense based on the vlan tags. (ex: vlan id2 = wifi, vlan id3 = wired). the AX1800 supports multiple SSIDs, but they're all be on the same vlan (ex 2). If you have NAT and DHCP enabled on the tp-link, then wifi (and any ethernet ports configured for "internet") will get DHCP from the tp-link and the TP link will perform NAT translation on traffic going to/from WAN port on the vlan id you set (ex 2). You'lll need to set up pfsense to provide DHCP for the other VLANs. If you disable NAT and DHCP on the TP-Link then you can let the pfsense handle DHCP for the tplink's "internet" VLAN as well.
I think the TP-Link configuration interface is only available on the "Internet" vlan.