What type is your Internet (WAN) connection ? PPPoE , Direct ? Have you observed the router logs for anything that may give you a hint ?

Try to connect to the device via wireless. If you have done a reset then your default wireless settings are on the sticker (on the back of the device).

Restoring to factory defaults every time after power outage is not normal router behavior. You can't store your default configuration in EPROM. Try to hard reset the router and re-configure form...

All the info you've gathered is correct and I would suggest to turn it on only if you need to, otherwise keep it OFF.

Correct, but once you turned off the DHCP server of the C5, it has to be connected to the ISP router, so your laptop would be able to receive an IP address (from the DHCP server of the ISP router).

@JTTigas Follow this guide for AP configuration (case 1) of the C5 router. (the link you followed is pretty much the same actually). If you already made these settings on C5, then connect C5 LAN...