Access second router from main router

Hi everyone! new openwrt user here, I installed openwrt on my redmi ac2100 yesterday.
I have another router with stock firmware, it is tenda f3. I'm using this router as extender in WISP mode, so it means router has different subnet, and yeah everything is working as expected.
And back to the topic, how can i access tenda f3 (admin page) from my pc which is connected to ac2100 (main router)?

AC2100 --> default gateway
F3 --> default gateway, has static ip from main router

my goal is any client who connected to (AC2100) are able to access (F3 admin panel).

You would need to ask on the tenda forums or support site since you need to modify the firewall behavior of this device (using the stock firmware). There isn’t anything you can do in the openwrt side to solve this.

What is the purpose of this device on your network? Maybe it can be setup in a different configuration if you don’t need two cascaded routers (for example, if the purpose is to use it as an access point, you can connect turn off the dhcp server, change the lan ip, and then connect it via the lan port to your main network)

Side note:
If you have two router running OpenWrt and connect them behind each other and the only thing you setup is changing the LAN-IP from one of the devices from to, then you can for example connect the WAN side from the to the LAN side of the When you then connect a computer to and enter in a web-browser you can change the configuration of the device. No additional configuration in OpenWrt is required in such cases.