AirPort extreme gen 6 set as Access point wireless

I want to use the airport upstairs, but I want to connect to the main router wireless

Unfortunately, this is not going to work. Apple designed their "wireless extend" feature to only work with other Apple routers. The AirPort Extreme will not be able to connect to a 3rd party router using a wireless connection.

Even if it could......it can't......there would be a loss of at least half of the potential speed on the network when you try to extend a network using only a wireless connection. This is true if you try to extend the network wirelessly using another device as well. So, an Ethernet connection is strongly recommended for best performance no matter what brand of "extender" you might be considering.

The only way that the AirPort can be configured to act as an extender for your network would be to establish a permanent wired Ethernet connection between the main router and the AirPort Extreme. Then, the AirPort can be set in Bridge Mode to act as a wireless access point providing a Wi-Fi signal using the same wireless network name and password as your main router.

If you want to give this a try, post back if need more information on how to set this up.

WiFi 6 is going to offer better performance. If the TP-Link extender also offers WPA3 security, then that is more secure than the WPA2 settings that the AirPort uses.

The AirPort was designed in 2013 and Apple never upgraded the hardware on the product, so it's frankly out of date compared to newer products that have come to the market in the last few years.