If somebody wants to set up auto-renewal in crontab w/ LetsEncrypt, the DigitalOcean docs have a pretty good approach for the cron job (assuming nginx):
# enter this
sudo crontab -e;
# and then add this line to the crontab
15 3 * * * /usr/bin/certbot renew --quiet --renew-hook "/bin/systemctl reload nginx"
Which means: At 3:15 AM every day, make Certbot check whether anything needs renewing. if anything updated, restart the reverse proxy service. Otherwise, do 0 things.