As a "First Aid" you should be able to reduce the ping interval, and the number of pings to fail before reboot. Unfortunately, I could only better direct you when running"official" openwrt.
Second, it looks like some speciality of your WAN-provider. You might check the RUTs log files for irregularities, regarding loss of WAN.
Some providers switches time out in case of no tcp-connection active for some time. Thus, also RUTs standard OS should have an option, to periodically establish a http(s) connection to a web service, to request some data. If not, write a small script yourself and schedule it using cron, every minute.