All EU868 channels are not used in ABP mode

I have a RAK3172 module and I’m trying to control it via AT commands. However, when I select EU868 band in ABP mode, I see that I can transmit packets only over channel 0,1 or 2. All received packets are coming from these channels. And I see that module only selects among these channels. I want to use all 8 available channels that are defined for EU868 band. I am also unable to send data when I set DR to 7 for FSK modulation. AT+SEND=1:1234 returns AT+ERROR after setting DR to 7. How can I use all channels?

Welcome to the forum @arifonder

By default in EU868 only the channels 0, 1 and 2 are open.
After successful join (on OTAA) the network server sends a list with the other 5 frequencies down to the RAK3172 and from there on the RAK3172 will use all 8 channels.
On ABP (not tested by me) I guess you need at least one uplink to the server to initiate the enabling of the other channels.

What is your LoRaWAN server? TTN, Chirpstack, RAK gateway internal LNS?

What is the version of your firmware on the RAk3172? Please check with AT+VER=?