I tried several approaches, but all failed, the specified limit_req does not apply. Those who understand proxies, please help or tell me where I can study the documentation. Thank you in advance.

It has different solution approaches,
Using NGINX and NGINX Plus, it is possible to limit:
The number of connections per key value (for example, per IP address)
The request rate per key value (the number of requests that are allowed to be processed during a second or minute)
The download speed for a connection
Note that IP addresses can be shared behind NAT devices, so limiting by IP address should be used judiciously.
It’s always good and advisable to google and check out a lot of articles when you face such problems because chances are, that someone else might have experienced same issue like yours.