Visit their site and pay the fee, which I think is around £25 or £30.
Then, you get a link to download DJI Fly App which has the FCC mod on it.
You then connect the modded DJI Fly app to your drone, and wait a few seconds.
Then check the...
My current configuration is:
RC2 - Latest FW
Mavic 4 Pro (512gb version) - 1.000.000 FW
Demon Tweaks FCC mod, on the RC2.
4G works an absolute treat. Just have to avoid the error of updating the drone FW, when the RC2 is practically screaming...