1. Find the systemd service file which starts Tvheadend (the first line of the output from
systemctl status tvheadend
will give this information)
2. Edit the file. Under the section [service] add the line:
ExecStartPre=/bin/sleep 10
this will add a 10 second delay to the Tvheadend startup.
3. Run
systemctl daemon-reload
to force systemd to re-read the modified file.