I am able to quite easily share my Internet through KDE using the Ethernet cable, and the plasma-nm widget in both Plasma 4 and 5, however, I am unable to share that connection properly to a DD-WRT router I bought, which allows me to flash to the latest version of DD-WRT.

My goal is to share the Internet access from my laptop to the router, which then acts as a hotspot so other devices can connect to it. This is necessary for my setup because the WiFi network I am using is based on MAC addresses, so it can be a hassle asking my building manager to register devices, or for guests who visit me.

Anyone with guides on how to set this up?

I am more or less a newbie to networking.

You're aware that DD-WRT allows you to spoof the MAC address of the router (ethernet and WiFi), ie. it can "have" the mac address of your notebook WiFi chip?

Whatever you're trying to do: unless your notebook is required for special hardware or software to connect the WAN, you're doing it wrongly

You naturally want to router to be connected to the WAN directly, so that you do not have to run the notebook permanently to have the router online.

Afaiu your "building manager" is providing a MAC filtering WiFi connection to the WAN
=> You want to run your dd-wrt router in client mode (wireless/basic settings) and clone your notebooks MAC address (setup/mac address clone)
See http://www.dd-wrt.com/wiki/index.php/Cl ... e_Wireless