Yes now I see your OS it's true. But openwrt has init.d scripts and have cron too 
https://openwrt.org/docs/techref/initscripts
Init.d for run rclone mount at restarts and make example a bash script what is runs from crontab to checks that rclone mount or command is running or not and start it if necessary. But it's only my idea to solve this, maybe somebody else have easier way 
And I see you wrote that rclone running few days fine and after shuts down. So you using your router for only rclone mount or you do commands too (copy, sync, etc) because if yes maybe your problem that rclone starts to eat too much memory so linux shut it down. Like here: https://forum.rclone.org/t/rclone-mount-setup-plex-server-reading-from-rclone-off-an-openwrt-router/8169
But If you would get better support then please correct your firs post with all the needed informations: os version, rclone mount or copy command, etc what is missing ----->