/etc/systemd/system/tailscale-cert.service

[Unit]
Description=Tailscale SSL Service Renewal
After=network.target
After=syslog.target

[Service]
Type=oneshot
User=root
Group=root
WorkingDirectory=/etc/ssl/private/
Environment="HOSTNAME=[your device's hostname]"
Environment="DOMAIN=[your tailnet].ts.net"
ExecStart=tailscale cert ${HOSTNAME}.${DOMAIN}

[Install]
WantedBy=multi-user.target

/etc/systemd/system/tailscale-cert.timer

[Unit]
Description=Renew Tailscale cert

[Timer]
OnCalendar=weekly
Unit=%i.service
Persistent=true

[Install]
WantedBy=timers.target