Kodi web interface is on port 8080, e.g. http://192.16.2.20:8080 <= replace with whatever the IP is of the LE device. There's a remote control in the web interface so you can navigaate around. If you don't understand how to use Kodi read Official Kodi Wiki
I use Ethernet .. because I rip discs with a high quality level and WiFi is rarely a reliable medium for streaming large files. I like reliable.
WiFi issues are always part environmental in addition to hardware/software. I can't explain why your pi does not reach AC speeds. I'm not sure anyone else can either. I don't expect WiFi performance from an RPi4 though; the design priority on a pi board is basic connectivity so that kids can do "online" things and develop fun stuff.