The "Wave 2" in the name is not marketing fluff. It ensures the router can handle the full 1 Gbps fiber connection that KT provides.
Turn off the 2.4 GHz and 5 GHz “Smart Connect” (band steering) in the advanced settings. While Smart Connect tries to put you on the best band, it often fails with older IoT devices. Create two separate SSIDs (e.g., “MyWiFi_5G” and “MyWiFi_2.4G”). kt giga wifi wave 2