@terziyski

I managed to get the WDS up with a few extra steps beyond those in the TP-Link guide.

1. Connect the TP-Link to a PC, using an Ethernet cable.
2. Login to the web-interface (in my case http://192.168.1.1 or http://tplinkmondem.net)
3. Go to Advanced > Network > LAN, configure the IP address. The repeater should be in the same subnet with the root router. If the IP of the root router is 192.168.0.1, the IP address of the repeater might be 192.168.0.2, for example. Click Save.
4. Note the new IP. If in future you wish to access the TP-Link's web interface either use a PC and an Ethernet cable or the Wifi of your root router. The web interface is less reliable -- if even reachable -- on its own Wifi, after setting it up as a repeater.
5. Go to Advanced\Wireless\Wireless Settings\Advanced Setting. In WDS, select Enable WDS Bridging. Click Save.
6. Click on Scan. Find the SSID of your root router and click on Connect. Click Save.
7. If the wifi of the root router is encrypted, please configure the security settings according to the root router's wifi encryption. Click Save.
8. Go to DHCP Settings and select disable. Click Save.
9. Reboot all: the root router, the repeater and every device you want to use the repeater's Wifi with.
10. Logon to the repeater's Wifi with each device you want to use and then set a static IP address. On Android 9, for example, after connecting to the repeater's Wifi, you open Settings, Connections and then tap on the repeater's Wifi. Then you open Advanced and switch IP Settings from DHCP to Static. You then fill in all the information. This reserves a static address for that particular network but not for any other network, so your device can switch between Wifi networks without creating IP issues.
11. Reboot everything one more time. Be patient, it may take a few minutes the first time for the bridge to start working.

Now, all of this works. But not that great. The speed of the repeater's Wifi is between 5 and 15Mbps, as compared to the 70Mbps of the root router's Wifi. Even when it is set right next to the root router. Also, the repeater doesn't like cold restarts of both the root router and the repeater after they've been powered off for a few minutes, and Disney+ stopped working on my Xiaomi Android TV set-top box.

? As to the speed issues, the WDS bridge currently runs on the 2,4Ghz bandwidth for sake of stability. I figure however that as the D7 has strong antennas it probably can establish a strong connection with the root router through some obstacles a mobile device might struggle with. On 5Ghz there should be less interference, if that is what is slowing down the connection. Is there anything else obvious to keep an eye on?

? Any idea why the WDS bridge might struggle when the root en repeater routers are both powered down for a few minutes? The IP of the repeater is set to 192.168.2.100, where it can be found on the root router, yet the root router assigns it the IP 192.168.2.102 according to its client list.

? As to Disney+ suddenly not working on the Xiaomi when it never had issues before, any ideas what that may be about? Disney+ support haven't been able to pin it down. Could it be something like MAC address of the root router having been hashed into my credentials for the Disney+ app?

  @Crike_Jr

Well, that worked.

The repeater's Wifi performance shot up to virtually 100% of the root router, when put next to it in 5Ghz mode. About 70Mbps down, 100Mbps up. Unfortunately, it lost a good deal of that performance when moved upstairs to the bedroom. The walls and doors did indeed negatively impact the performance siginificantly. 20Mbps down, 40Mbps up. Moved to the attic, performance in the bedroom went up to 35Mbps down, 50Mbps up. Thanks to the thinner planks that span the attic floor and stairwell to the lower floor, rather than the solid walls and door of the bedroom.

The improved performance also resolved the connection to Disney+, or maybe the repeated restarts of the Xiaomi Android TV box did. Either way, that issue is resolved as well. Finally, changing over the repeater to 5Ghz from 2,4Ghz went without any issues. Reboot the root router, reboot the repeater and make sure when you login to the repeater's Wifi to set it to a static address again.

Thanks @terziyski for looking over my shoulder.