[stomachmonkey]

Consumer routers by default auto assign IP addresses.

They broadcast themselves as DHCP servers.

Devices look for them to pick up an address lease.

Two routers on the same network acting as DHCP servers is no bueno as they will curb stomp each other.

You want to login into the TP Link back end and set it to Bridge Mode so it stops acting as a DHCP server/ router. I would also switch it from dynamic IP addressing to using a static address.

Your primary router is probably something like 192.168.0.1 or 192.168.1.1. Check the Connected Devices page on the primary router to see what's currently attached and what IP addresses they use. Assign the TP Link an address not currently in use.

You also have WAN and LAN (Wide Area Network / Local Area Network) ports on both routers. This is where consumer routers trip people up.

Think of WAN as everything before the primary router and LAN as everything after or everything outside your house vs everything inside your house.

The natural inclination is the primary router is connected to the WAN, in consumer broadband it almost never is. There is another device like an ONT between where the service enters your home and the primary router itself which is the WAN device.

Long story short a TP Link LAN port should be connected to ASUS LAN port.

Some routers like your primary allow you to login over WIFI, some will only provide the back end over ethernet. To get to it open a browser window and try the 192.168.01. /1.1 addresses. They are default for routers but I have seen 192.168.0.254.

If you've never done this then the user / pass is the factory default.

User is almost always simply, admin

Pass should be on the back / bottom of the router.