You can always set up netcat UDP port listening for debugging in any linux server:
nc -u -l -p 3333
and try to configure your gateway to send packages to this server in addition to TTN endpoint...
it works very well with Lorank 8 gateway, because you can set up this UDP listener locally inside the gateway.
You will be able to get fast feedback - see if the packages are received, but only downside is that you can not decrypt the data.