i have almost 100 pcs of 901nd v5 running openwrt. i want to restart them daily to increase stability.
Best answer would be to figure out why they're unstable (other than the fact that they don't have enough RAM). Guessing RAM exhaustion, I'd trim down their workload so they are more stable.
cron job -- but be aware that they will restart to a time in the past, so you need to be careful not to re-trigger the reset when they come back up again.
See, for example https://wiki.openwrt.org/doc/howto/cron#periodic_reboot_of_a_router and