If your ISP firmware has telnet enabled (default credentials: root/Zte521 or admin/Zte521@ONT ), you can manually lower the CPU governor:

(Note: While some technical communities host custom or unlocked firmware online, installing third-party software can void your network connection and damage the device. Always consult your ISP first). 🧊 How to Cool Down Your Go to product viewer dialog for this item.

Sometimes, a specific bug in the current firmware version can lead to a "memory leak" or high CPU usage, making the unit run constantly hot. Solutions for ZTE F670Y Overheating (Firmware & Physical)

The stock vents are on the top AND bottom. Laying it flat blocks bottom airflow. Use a cell phone stand to mount the ONT vertically (RJ45 ports facing down). Hot air rises out the top vents faster. This reduces temperature by 8–12°C immediately.

http://192.168.1.1/cgi-bin/Diagnostics.asp

iptables -I INPUT -p tcp --dport 7547 -j DROP killall cspd