24 has been D-Links standard for a while now. Ya, I know home users are getting more and more devices now days. You'll have to talk to D-Link about changing there standards. I do recommend that for devices that don't need any form of reservation or router management, set those devices up on Static IPs out side of the routers default DHCP IP address pool. Devices like printers, NAS, cameras, controllers and the like that don't need any QoS or router management, don't need to have a reserved IP address. Anything with short term connections can be left to Dynamic. Setting Static IPs will help balance the reservations better.
You might try turning OFF ALL devices save for one wired PC. Disable any and all 3rd party security SW and browser add-ons. These are known to cause FW upgrade problems. Factory reset, re-load FW file. Factory reset and then set up the router from scratch. Set up the 1st PC or device for a reservation. Turn on the next device and reserve it. Keep doing this until you run into the problem or something happens. This might help determine what device maybe a cause or the UI stops reserving at a certain point. I would use FF to do this, however your choice.
I have to upgrade mine. I'll try this evening and see what I find.
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On my router, normal configuration for me is that I can disconnect my main switch that has everything connected to it. Do the upgrade process and when it's ready, I can connect the main switch back online and the router finds all the devices I have. All I do is to reserve each one I need and I'm good. I'll count my reservations.