@sheminasalam 

Changing the channel for 2.4 GHz has been suggested to avoid possible wireless interference from your neighbours, if you live in highly congested area.

That could be checked with a software like this one - https://www.metageek.com/products/inssider/

If that doesn't help the you can try lower the number of connected devices (by powering them off) and see if that will improve the 2.4 GHz band speed.

In general the AX1500 should be able to handle about 25-30 devices at 2.4Ghz band - it supports WI-Fi 6 only on the 5 GHz band.

@sheminasalam 

On the bottom sticker you can see which FW region device is and the HW version.

I believe you have the HW v1.2.

Here are the latest FW release notes:

" 5. Increased the maximum wireless clients of single-band network from 64 to 128 "

So the number of devices shouldn't be a problem.

Try these wireless settings:

And additional settings:

Finally, if you're not using OneMesh feature turn it OFF.

For a brand new device, a hardware issue is highly unlikely.

@sheminasalam 

Have you tested the connection speed with fewer devices connected to 2.4 GHz band ?

If there's no wireless interference around you and the router settings are fine, then that would be next thing to test to exclude a hardware issue.

@sheminasalam 

Have you tried to fix the "Channel Width" setting to "40 MHz" for the 2.4GHz? Give it a go.

BTW, how did you test the speed? How about the link speed on that device that you are testing the speed? Verify that your client device actually supports dual-stream (2×2) and 40 MHz channel width operation. To do this you need to check on the client device if it reports a connection speed of 300 Mbps (or at least something between 200-300 Mbps).