There's no issue with the Roku devices and the device with the Roku app being on different WiFi bands (2.4 or 5 GHz) or even a wired connection with a Roku Ultra. They will still be "seen" and allow a connection. I have had an Ultra with a wired connection, a Premiere on 2.4GHz, and my phone on 5GHz, and the app on the phone could see and connect to all of them. Bluetooth being on/off on the app device doesn't matter, as no Roku supports a network connection via Bluetooth.
But the network in question must meet certain requirements. First, the IP address range the network uses must be within what is considered a 'private' network. Most home networks will use 192.168.x.x, with the third digit being anything between 0-254. The final digit must be different from every other device on the network. These address assignments are controlled by what is called a DHCP server, and is most commonly part of the network router operating system.
The second important thing is the WiFi network must not be using a security function that blocks connected devices from seeing other connected devices. This is always found in networks operated by hotels, schools, and other semi-public uses. And on networks like that you want that security enabled. On a home network, it's really not necessary and will block things like the Roku app from working.
Dan
Roku Community Streaming Expert
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