I mentioned before that the Gateway was working, but that was not true. I was lost for several hours as I got a few messages from my nodes, but after some time communication stopped. I noticed that in the TTN gateway traffic I received a few messages in the 867MHz band with a very low rssi although the node/gateway were a few cm away. My local configuration should be US915 (somehow the gateway captured 915 messages when it was configured in 867).
I solved the problem by changing global_config.json file in /opt/ttn-gateway/bin with the contents of file US-global_config.json (https://github.com/TheThingsNetwork/gateway-conf). A similar problem is found here RAK831 config issue?
Disclaimer: I don’t know if making this change affects something else (i’m new with this stuff), so any advice from the community is highly appreciated.