Use TP-Link Archer C7 as extra switch and wireless AP

I am definitely not an expert when it comes to home networking. The current wireless setup in my home is as follows: I have one device from the inteet provider in the house that acts as router, switch and wireless AP (range 192.168.0.*). That's it, plain simple, and it is quite satisfactory in the house. However, the possible number of settings on this device is quite limited (e.g. I cannot configure a DHCP range, only a subnet)...

Today I tried to extend the wifi range to the garden terrace. I bought a TP-Link Archer C7 V2 router and connected the in-house device and the Archer through a cable both on the LAN ports. I disabled DHCP and NAT on the Archer and configured a static LAN IP for it. This seemed to work over a cable. Then I wanted to have the same wireless network SSID both in house and in the garden. So I configured the exact same wireless settings on both devices (same SSID, same key, same security settings, but a different channel).

When I am in the garden, everything works fine. I get an IP address in the correct 192.168.0.* range, I can access the Archer through its configured static IP address, and I have inteet access. Same still in the house.

However, when I go from the house to the garden while connected to the wifi, my computer picks up the right AP, but something goes wrong with the DHCP, I think. Inteet access is gone, and my computer gets a self-assigned IP address in the 169.etc range and no gateway, not entirely correct... It is only after I configure a static IP for my computer that everything works. After that I can switch back to DHCP.

Some thoughts on this? What have I done wrong?

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