How to change router's MAC address - Router UI does not have an option for this.

Wondering if anyone knows how to change/spoof a router's MAC address when the router's built-in control panel UI has no option for this.

Are custom firmware's available? Would it be possible to extract the router's firmware, output it as a readable coding language and find where such information is stored or authorised to be broadcast onto the LAN and WAN?

I'm stuck with a very basic router that has few options, most of which are ineffective from a security/privacy perspective.