Airport Extreme ethernet connection to AirPort Express via LAN or WAN?

I know this is very old equipment.... but it still works. Have been using an AirPort Extreme (7.9.1 version) for several years with no issues. Due to a dead zone in a corner of the house, I now need to extend the WiFi signal to that area. I did set this AirPort Express (7.8.1 version) up using "Create a wireless network" in Airport Utility. This is the set up currently: My Cable Modem is connected to the AirPort Extreme WAN port - this is connected to the AirPort Express LAN port via ethernet cable. It is working with WiFi speeds up to 93mbps on the Mac that is sitting next to the AirPort Express. (When the Mac was connected directly via the the ethernet cable I was at 608 mbps and I knew I would loose speed via the Express wireless, but I need WiFi in that area of the house) BUT, I am wondering if I have set this up wrong by using the LAN port on the Express. Was I supposed to use the WAN port? What is the difference? Thanks for any help or suggestions.