Cron jobs can surely be used in openwrt. But the question is if you have DNSCrypt-proxy installed on your Openwrt installation. If yes and if you still want to restart it with a cronjob you just need to adapt the command to reinstall it.
Generally there's no reason to restart services that work properly.
If you have specific issues, it's best to discuss them before applying workarounds.
Then readup the doc I linked above and adapt the command. Most likely it be /etc/init.d/dnscrypt-proxy restart assuming that it is a proper service on openwrt.