The 3920 model is the Roku Premiere. Both the Roku Express and the Roku Premiere only operate on the 2.4gz band of your network, so seems like you have everything working correctly. [single/mono band devices]
The Roku Streaming Sticks, the Ultra, Streambar, and Soundbar are the Roku models capable of using the 2.4gz or 5gz bands of the network. [dual-band devices]
The single band devices can only "see" and use the 2.4gz band.
The dual-band devices can "see" both the 2.4gz and 5gz bands and use either the 2.4gz or 5gz band.
Hope that helps clear it up for you.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102