@FloSun 

TP-Link Archer uses OneMesh technology for WiFi mesh, which is not the same as Deco mesh. You can't make one seamless WiFi mesh from both.

If you can, replace Deco P9 with TP-Link OneMesh capable devices. If you can't do that, there are things you can do to improve what you have.

First, change your Deco mesh to Access Point mode. Switching a Deco to Access Point (AP) Mode. After you've done that, you will let Archer manage your home network (IP address allocation and such). 

With Archer router in charge of that, you can seamlessly use Archer Ethernet ports to connect anything to them, and these wired devices will be accessible through Deco WiFi. For example, if you have printer with Ethernet port, you can now connect it to Archer and can print on it from laptop wirelessly connected to Deco mesh. It will be seamless.

Another benefit of Access Point mode is you don't have to wire Main Deco directly to Archer. If your house is Ethernet wired, you can now move Main Deco anywhere there is Ethernet cable from your home network. That may allow to position three Deco nodes for better WiFi coverage.

Also, there are some wireless devices, such as older security cameras, which do not work well with WiFi mesh. If you happened to have such devices and they are in WiFi range from Archer, you won't need to mess with Deco settings trying to make them work. Just set Archer WiFi with different SSID, enable WiFi on Archer and connect these wireless devices to Archer SSID.