In the app you can add and link everything (to the gateway/hub), even the sockets, but those sockets are IMHO not dimmable.
I also believe that it is possible to link lamps with the dimmer or remote control directly (without a gateway) but than you won’t be able to control them with HomeAssistant or by the app.
You need to connect the ikea dimmer with your socket, then add both to the gateway. Then you can add the gateway to HA. Howto: it is in the booklet provided with the dimmer and socket.
If you use the Ikea tradfri gateway, there is no other way to connect the socket to the gateway. You need to use a remote. Stupid system, but that is the Ikea way.
Okay, it took some time (I did not have time to respond). Anyhow,
To give an update, yes you can only see the battery state from the power button, I fixed the switch.
So I ordered an ConBee II Zigbee USB-Stick to connect to the power button.