Enabling external booting on a fusion drive Mac

I am trying to change the secure boot settings so that I can allow booting from an external disk. Normally to do this you would go into recovery mode with command + r and then change the settings. When I try to do this, however, my mac decides to boot up using recovery mode with internet, in which I cannot change the secure boot settings from. After doing a little research, this seems to be because my Mac has a fusion drive, which is causing the regular recovery mode to not start up. My question is: how do I get into regular recovery mode on a fusion drive Mac? If I can’t, how do I change the secure boot settings without going into recovery mode?