The clock source on the Focusrite should be internal.
Sounds like you’re already doing this, but the ES-8 USB should be disconnected if you want to use it standalone. Also it only switches into standalone after a power cycle, not just when you unplug the USB.
I would confirm your Focusrite settings first, by taking an ADAT cable out of the Focusrite output and back into its own input. Then try to send 8 channels of audio out of the Focusrite and back in.