@GuyCai 

Apple devices generally speaking for 2.4 GHz have always worked better on the higher channels.  Apple even has a help article that describes the best settings for Apple devices on 2.4GHz.  Trying this may improve service.  We have also noticed some older legacy devices have network card that simply do not preform well with AX on the 2.4GHz enabled.  This would be due to the software and in some cases the hard of the device's adapter and we would not be able to resolve taht other than setting the band to N only or at least removing AX.  If that is the case contacting that devices manufacture to see if an update can be applied to make the device more compatible with AX may be possible. 

2.4GHz channel width

Channel width controls how large of a "pipe" 'is available to transfer data. However, larger channels are more subject to interference and more likely to interfere with other devices. A 40MHz channel is sometimes called a wide channel, and a 20MHz channel is a narrow channel.

Set to: 20MHz

Use 20MHz channels in the 2.4GHz band. Using 40MHz channels in the 2.4GHz band can cause performance and reliability issues with your network, especially in the presence of other Wi-Fi networks and other 2.4GHz devices. A 40MHz channel might also cause interference and issues with other devices that use this band, such as Bluetooth devices, cordless phones, and neighboring Wi-Fi networks. Routers that don't support 40MHz channels in the 2.4GHz band do support 20MHz channels.