I'm running the DIR-655 A4 with the 1.36 firmware. I usually view the router's menus with Firefox. If you change the page style from "basic page style" to "no style" when viewing the Network Filter screen under advanced, you'll see that there are buttons to filter wireless and/or wired. I assume that they'd actually work.
As others have noted, with MAC filtering on and "allowing" addresses in the list access to the network, it's not too hard to get to the 24, even though only a handful of the devices may actually be accessing the network at any particular time. Consider that a laptop has two MAC addresses (assuming you'd have reason to connect via Ethernet) then you have TVs, DVD players, Video game consoles, phones, tablets, etc. and it's inconvenient to have to edit the list constantly based on whatever devices are in use. I have 22 of the 24 slots allocated now.
One issue I have is that i use MAC filtering as part of may limiting hours of use to the network, because my kids figured out that they could change the MAC address to bypass the schedules, and they have wired access as well as wireless.