The router does broadcast 2.4, but my Smart Home App that is required for the Smart Plug is not finding it.
Someone else may need to step in about this since I've not had experience with that specific kind of thing.
I do have a ton of devices - several handheld and console gaming devices (all Nintendos), 5 phones, 3 tablets, 4 computers, a couple TVs, 4 Fire Sticks for TVs, 2 Echos, one one Ring doorbell device. All are connected by either 2.4 or 5 GHz. No issues with seeing any device that needs to be seen by any other device on the other band. Because the router itself is the common connection. Even for the time I had one computer directly connected to the router with the ethernet cable.
I'm amazed my router can even keep up, but it chugs right along. Less than half of those devices actually have a constant data flow. Most are just connected to be connected.
But no smart plugs anywhere for me, so I don't know if there could be some particular condition that kind of device requires.
However, I still don't think that should matter. If the app is on the phone, and the phone is on the network (regardless of which band), the app should see the smart plug as long as the smart plug is on the network. Aren't these things supposed to work even if you leave the house and the phone is only on your cellular network, once they initially see and connect with each other?
Have you tried just using the app to find the plug? Is the app giving you some kind of error message about which band the phone is connected to?