@nyonyo 

Hi,

Can you post the brands and model numbers of those devices?

Until not so long ago there were still lower-end phones and tablets available that were only capable of 72 Mbps link speed, which resulted in around 40 Mbps of usable real-world speed.

Furthermore, are there a lot of other 2.4 GHz Wi-Fi networks in your neighborhood? When you search for Wi-Fi networks on your phone or computer, how long is the list of available networks?

  @woozle Most of my network devices are tp-link, archer c80, archer c54, RE200, RE305, and so on, all 2.4GHz give me around 40, Yes the link speed I get in my phone (Samsung A51) is 72 for 2.4GHz N300 and 65 for N150, but the interesting thing is I get full 54Mbps on WA5110G. As for the 5GHz I only get 86, no matter it is AC433 or AC1200

When I search around my house there are only a few Wifi, about 5, all are 2.4GHz, no 5GHz detected, all those 2.4GHz have much weaker signal than mine and use different channel