@222jacky 

As I understand, you are saying that the router restarts or stops working when it is overloaded by a speed test?

- Maybe its processor is overheating? Make sure the router is properly ventilated or cooled!

- Perhaps the power supply does not provide enough power when the router is overloaded? Try to replace it with power supply providing a minimum of 12V/2A stable!

@bernado64 

Thank you for your reply ,I will try another power supply and try the test again！

@222jacky 

My router is arranged vertically, maybe this gives it good ventilation and better 4G LTE reception with the internal antennas?

The standard power supply has been replaced by a mini UPS to get 24/7 operations, without risk of a power outage (the battery backup should be more than 4 hours).

More infos in https://community.tp-link.com/en/home/forum/topic/252076