Then start testing it ..
rc.local
#!/bin/sh
random_mac(){ printf "%012x" $(( 0x$(hexdump -n6 -ve '/1 "%02X"' /dev/urandom) & 0xFCFFFFFFFFFF )) | sed 's/../:&/g;s/^://';}
uci set network.wan.macaddr $(random_mac)
uci commit
edit: it might need a commit at the end