Almost, but not quite. Your code doesn’t add any additional TTN-specific channels, but then you’ll still have the 3 default channels. These are needed in each network for OTAA joins of LoRaWAN-compliant devices that need to know some basic settings that are generic for all networks.
I don’t see you disable the other two defaults anywhere. The following just enables a channel that is already enabled by default, but you’ll need a few LMIC_disableChannel instead:
That’s also a problem of using a single-channel forwarder rather than a true gateway: nodes that are adjusted for such forwarder will often use one of the default channels, making those default channels being used far more often than the other channels a network might support.
Also, are you sure 20 is allowed in your region?
(For your next post please also see How do I format my forum post? [HowTo] And please add all details in your first post next time. Thanks.)