Class-B is disabled by default on firmware 4.0.x

You can activate the feature by clicking the Support class B Enable checkbox in the Radio Configuration menu of your gateway:

1. From the dashboard, select Management > Gateways
2. select your gateway EUI
3. select Configurations > Radio

In the bottom of the page (scroll the page down), you should see:

If this feature is not visible, you must check your radio configuration:

1. select “USE RAW EDITOR” button in the right side of the page below the graph
2. insert the following piece of code at the end of the configuration file (don't forget the first coma!):

    ,
    "gateway_conf": {
        "beacon_enable": false,
        "beacon_period": 128,
        "beacon_freq_hz": 869525000,
        "beacon_freq_nb": 1,
        "beacon_freq_step": 0,
        "beacon_datarate": 9,
        "beacon_bw_hz": 125000,
        "beacon_power": 14,
        "beacon_infodesc": 0,
        "ref_latitude": 0,
        "ref_longitude": 0
    }

This piece of code must be customized according to your country. Refer to the LoRaWan Regional Parameters specifications that you can find here: https://lora-alliance.org/resource-hub/lorawanr-regional-parameters-v102rb

Your page should look like as following:

Then, perform the following steps:

1. Submit the changes to your gateway by selecting the SEND button. The gateway will reboot and apply the new radio configuration.
   
2. Select the “USE VISUAL EDITOR” button (just below the graph in the right side of the page)
   
3. Check that the feature “Support class B” is visible
   
4. Check the box to enable class B feature
   
5. Click the SEND button once again. The packet forwarder will restart applying the B-class configuration.

Simply, uncheck the Support class B Enable box in the Radio Configuration menu of your gateway.