Why hub 5 does not connect to devices that support 2.4 gb like my s.art lights ?

Many IoTs have low cost Wi-Fi adapters and incomplete driver support, this is why they don't work with the Wi-Fi 6 service of the Hub 5. 

Here are some practical options :

1) Run the Hub 5 in modem mode and add a decent Wi-Fi router that allows you to control what 802.11b/g/n/ax modes are operated on the 2.4GHz band. Make sure ax can be disabled.

2) Use a Wireless Repeater that outputs Wi-Fi 4 & Wi-Fi 5 as a bridge to the IoTs
     ( TP-Link RE450 AC1750 is been used by several customers and works with the Hub 5 in this role )

3) Add a basic Wireless Access Point just to support the 2.4GHz IoT ( these are inexpensive )

4) Replace the Hub 5 with a Hub 4 that has the more compatible 2.4GHz Wi-Fi 4 and 5GHz Wi-Fi 5