You can, in which case you’d need to register the node with whatever on-gateway or cloud based network server you were using.
In the official LoRaWAN spec, nodes in multi-subband regions like the US are required to try to join on random selections across all possible subbands, because officially they can’t “know” which subband the network is going to be on. This is rather inefficient, since only a minority of the packets would be picked up, so often people cheat the spec and preconfigure the node to the subband on which their gateways are listening.
My guess would be that even if you’re not connecting it to TTN, your gateway is still configured for the second subband, channels 8-15, though for a private network you could chose to set it to a different one, and might want to if there were already a lot of local usage of the default.
Also make sure the public / private network switch which controls the syncword is set the same way for both the gateway and the node. Most are set to use the public syncword, even if they’re private, but the setting absolutely has to match.