I've been struggling a bit with the Pulse-Eight USB-CEC adapter.
At present between trying things out I unplug all cables and put things neatly away. I just set things up again and the FF/Rewind/Play/Stop buttons did not work. I tried various things and found that rebooting LibreElec made them work. I then tried pulling the power lead from the Dell and left it a while, plugged power back in, started the Dell and all buttons worked.
I did notice that with a reboot I get the "connecting to" message with the first boot after plugging cables in I only get the connected to tv message. Is it possible that LibreElec is trying to set CEC up to early?