I am in the exact same situation. I have also tried changing some other values in the se-identity file to match TTN (such as the secret keys) but that didn’t work.
I also tried creating a new device on TTN to get fresh keys but the result was the same.
Finally I tried using ABP by following a tutorial I found online but that did not work either.
I know my gateway works fine because I have another device sending data to TTN through it with no issues.