Don't know if you already found the solution, but "citadel" instead of "umbrel" should be the right directory name?
So the commands have to look like these?
sed -i "s/\[Application Options\]/\[Application Options\]\nreset-wallet-transactions=true/g;" ~/citadel/lnd/lnd.conf && sudo reboot
and Step 3:
sed -i "s/reset-wallet-transactions=true//g;" ~/citadel/lnd/lnd.conf && sudo reboot
--- Not tested, but looks logically ---