Having done multiple hotspot systems, based on openwrt, (but not on RUTs,) it looks like failover and "Captive Portal" interfere with each other on your device. Which is not surprising, as both usually "mess around" with the firewall rules.
Especially, as I suspect, that the RUT uses "coova-chilli" as Captive Portal software. Which is rather complicated to be set up properly, even without failover of WAN.
In your case, I suggest to write a short script, to intercept/poll "connection loss" , and to do an auto-reboot, in case "connection up" again.
You can intercept the log messages (logread -f) and filter for "connection" up, OR poll "mwan3 status". (May be, something different used on RUTs flavour of openwrt, to do the failover).