Roku devices cannot be found under same internet with bluetooth on

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.