Why deco m9 plus the wifi speed far diffrent with the deco speed direct connect to router ? my deco setting as AP mode and the deco just right beside me. my device direct connect to main deco via wifi

Hi, I have 2 deco m9 plus (One deco to other deco maybe around 20 feet) with fast roaming enable, it working perfectly if the device connected to 5Ghz but when the other device connected to 2.4Ghz the