Diagnosis
It's probably not defective. The RT-AX3000P has internal antennas, and its signal can be heavily impacted by placement, or by a default setting that caps transmit power for regional compliance.
How to Fix It
Placement is key. Get it up high, in the open, and away from large metal objects, brick walls, or other electronics like cordless phone bases and microwaves. Even being tucked inside a media cabinet can kill performance.
If That Didn't Work
You can try increasing the transmit power. In the web interface, go to Wireless > Professional tab for each band. Look for 'Transmit Power' or 'Tx Power Adjustment'. Change it from 'Auto' to 'Performance' or the highest percentage available (like 100%). This isn't a magic bullet, but it can help.
Heads Up
The 5GHz band has shorter range but higher speed. If you're testing range on the 5GHz band, that's expected. For better coverage at a distance, make sure your devices are connecting to the 2.4GHz network, which penetrates walls much better.