How to get remote access to a subnet with a dynamic IP of the main router?

Question

Remote access

Faced a problem:
There is a main router (Sagemcom [email protected] 2804 v7), through which it distributes the Internet on the 192.168.1 network. * and the external IP address is dynamic. NOC Asus RT-N12 is connected to the main router (I don’t know the login and password from the web interface), which takes the Internet from the main router and distributes it to its subnet like 10. *. *. *
Question: What are the options for remote access to a subnet, for example at 10.20.30.40? You need to connect to the web interface of the device remotely.
PS I have access to a Timweaver machine that is on this subnet.
I would be grateful for a detailed answer! Help out!