Important is that you update your initramfs with the command after you edited the dropbear initramfs config and or you copied the key over.
For the client it is important to define 2 different known hosts files since the same host will have 2 different host keys, 1 when encrypted with dropbear, and 1 when operational with (usually) sshd.
Also you need to use root when you connect to your server to unlock it. No other user will work with the default setup.