@EddieFiVE If you are concert about the double NAT in this scenario you can try to find the NAT option switch in: Advanced=>System Tools=>System Parameters The settings if it is present in C5400X...

What about the Static LAGG option, do you have that one in you NAS settings (it maybe named like Load Ballance for ex.) ?

Have you tried to access the router remotely from different devices to isolate any browser issues on your computer ? Downgrade to a previous FW version (w/o HTTPS) is an option that's worth a try too...

@peter_jai You can try this procedure for un-brick your device.

Have you observed the router logs during these reboots, anything unusual ? CPU/Memory utilization via GUI => Advanced=>Status ?

@peter_jai Usually the TFTP method works for reverting back to the OEM FW from OpenWRT. If you tried everything else so far, that's the way to go.