Your router's placement is probably causing the weak signal. Physical obstructions and interference are the main culprits.
First, you need to move it. Place the router in a centralized area for maximum coverage. Keep it away from metal objects and direct sunlight. Then, you should check for interference. Move the router away from things like microwaves, cordless phones, Bluetooth devices, refrigerators, fluorescent lights, or heavy-duty motors.
Finally, orient the three antennas properly. Position each one at a 90-degree angle relative to the others.
IF THAT DIDN'T WORK: Make sure you're not connected to a 2.4GHz-only device network that's set to 20MHz or 802.11g mode, as that can limit performance. Also, always update to the latest firmware from the ASUS website.