It depends on your setup. P9 supports the bridge mode according to the instructions. However, a wired connection is necessary for the main unit, a pure repeater function is not supported. See https://www.tp-link.com/us...

This means you can switch off the WLAN from the Archer, attach a Deco P9 unit and output the WLAN there via Bridge Mode (pure WLAN access point). You then place the second Deco P9 somewhere where it still has a good connection to the main unit. Depending on your needs for the WLAN, this setup can also be achieved with other products (e.g. Ubiquiti).
If you are looking for a pure WLAN repeater, i.e. you want to keep your WLAN on the Archer and simply boost the signal in one place, you need to look for a repeater, the Deco P9 cannot do this.