I presume you're talking about the printer's own web-based control panel, rather than the CUPS front-end, (https://localhost:631 ) ?
I can access my (HP) printer's web panel from System Preferences > Printers & Scanners.
If I select the print queue, then click on "Options & Supplies", I get a button: "Show Printer web page".
If you know what your printer's IP address is, then you should also be able to use that in Safari as a URL, e.g. https://192.168.1.24
You can often get the IP address by printing a configuration page.
Some printer drivers come with utility apps that connect to the printer and provide status and configuration. These may also have a link to the printer's internal web interface.
Once you've got to the web page, bookmark it in your browser, to access it easily again.
Most functions, such as Sleep Delay, can also be accessed from the physical control panel, though the 'user experience' of cycling through options and hierarchies can be overwhelming.