tring to change nix substituters, with --option substituters, and ~/.config/nix/nix.conf file, however nixos-rebuild it will continually use the previous substituters.
I’m having the same problem… 
Are you a trusted user? I think nix ignores substituters silently if you aren’t a trusted user
I believe I am, I have this line in my config file
nix.settings.trusted-users = [ username ];