How to set up Nginx proxy on Openwrt on two internal servers with different sites?

Question

Nginx

There is a router with Openwrt, there is one web server with a website on the network. Now it's just configured to forward port 80 requests to the web server.
You need to connect another server with your site. Google suggested that you need to put Nginx in front of the servers and it will resolve requests to the servers.
But there is no normal simple example how to do it. Everywhere they offer instructions like nginx gives statics, PHP processes Apache, go some container with docker, it’s not clear where ...
Please help me write the config so that just
site1 = redirect to 192.168.137.2,
site2 = redirect to 192.168.137.3.
No problems with static / dynamics
I understand correctly that if nginx is also installed on the internal servers, then putting another one in front of them is nothing to worry about? :)
// don't throw slippers, I haven't learned more than ready-made wamp assemblies yet :)