How to forward a web form from a device to the Internet?

Question

Network administration

Good day everyone!
Everything is very simple. There are:
1. Good old (dusty) router Asus WL-500gP V2.
2. A device (web server, practically) that collects information via a serial protocol, systematizes it, and issues it in the form xxx.xxx.xxx.xxx/pages/bla-bla-bla
3. There will be a USB 4G modem, with a fixed IP.
The router is configured for WiFi "test1", which I connect to, enter the local IP xxx.xxx.xxx.xxx in the browser , and get the page. I want the same, from the Internet. I will get a fixed IP, how can I bind it to xxx.xxx.xxx.xxx/pages/bla-bla-bla? Or to the test domain (test1.ru = local xxx.xxx.xxx.xxx/pages/bla-bla-bla)?
In parallel with this, there is a desire to raise a web server on Asus. Not prevent?
It's just that I have never dealt with such tasks, in principle, it is clear that all this is realizable, but from what end should we approach?
Thanks in advance.

Answer 1

Here is the port forwarding Asus WL-500gP V2
Only instead of the camera there will be your device. In order not to wave the built-in web server, broadcast a non-standard port, for example, external XXXX: 81 to internal YYYY: 80