@Spyros, some other people have noticed similar behavior with the 5GHz band crashing on a different line of hardware over in a different thread. This might or might not be the same bug, but a practical workaround until the cause of the bug is discovered may just be to put a script in your Scheduled Task (crontab) section to reboot the router at, say, 4:30am each day. (See the following method: https://openwrt.org/docs/guide-user/base-system/cron#references )
Obviously, people who require literally zero downtime will have to find some other solution.
See here for the relevant discussion: Users needed to test Wi-Fi stability on Linksys WRT3200ACM & WRT32X on OpenWrt 21.02 - #675 by WildByDesign