Yes, LAN ports are in reverse order. WAN port is assigned correctly.
I succeeded with the VLAN configuration yesterday. It was quite well documented in several blogs how to set up VLANs on OpenWRT. That made it easier to figure out how to realize it.
My goal was to separate guest traffic from my regular home network. That was the reason why I installed OpenWRT on the Netgear R6220 as this feature is not supported by stock firmware.
This is my setup:

This might look a bit strange. Let me quickly explain what I did:
The Fritzbox is my router to the internet. It doesn't support VLANs but provides a dedicated LAN port for guests. That's why the Fritzbox has two connections to my switch. One connection is used for my regular network traffic and the second for guest traffic. The Fritzbox provides different DHCP ranges on both interfaces and separates the traffic.
My switch (a Netgear GS724Tv4) has all required VLAN options out of the box. By default all ports are in VLAN 1 (untagged). I had to remove the guest LAN port from Fritzbox from VLAN 1 and added it to VLAN 100 (untagged). To support traffic flowing through that port I had to set the port with PVID 100. From that point in time the switch had two separate VLANs - VLAN 1 for my regular network traffic and VLAN 100 for guest traffic.
Towards my R6220s which I use as Access Points on each floor in our house I configured a VLAN trunk - meaning VLAN 1 and 100 tagged to the ports on my switch.
Now I am coming to the R6220 setup. I created a VLAN 100 in the switch configuration and tagged the ports CPU and WAN - all others "off" as I don't want guests to use my Ethernet ports. I configured VLAN 1 to be tagged on CPU and WAN as well. I am still using VLAN 2 for the LAN ports (untagged) and CPU tagged. I could merge it to VLAN 1 but didn't change it so far:

Next step was the WIFI configuration. This is pretty much straight forward. Just created two SSIDs for each radio interface (I had to assign the radio frequency manually):

Next part was the interface configuration. I use two bridges and dropped the pre-defined WAN interfaces. The bridges merge traffic of my home network (LAN ports, WIFI 2.4, WIFI 5G, WAN) and guest traffic (WIFI 2.4 Guest, WIFI 5G Guest, WAN):

If anybody wants more detailed information on the configuration just ask. So far it works stable and I am very happy that I could re-use my R6220s for the guest VLAN.