Alex,
In that case, the gateway is listening on the 3 default channels for the EU868 frequency band and sends down the center channel for the other 5 in the join accept. You can see the configured center channel in the AEP web UI on the LoRa configuration page.
Once you have the center frequency for the extra channels, you can determine the frequency for each of the 5 channels and configure your endpoint to use them as well as the default 3 channels.
Cheers,
Mike