There's no requirement for you to use the recommended network, and once you connect to your desired network it stays there. I've never seen a Roku with any pop up message suggesting I change the WiFi network I'm using. The Roku would have no way of knowing if the other network is even available for your use (it might be a neighbor), so it really can't make an accurate suggestion for that.
One reason it might be recommending the 5 GHz radio, despite it reporting a lower signal level, is that the 5 GHz band is less crowded, and even with a lower signal level it can stream at a higher and likely more reliable speed. In tests with my Ultra 4800, which is within five feet of my access point, I get close to double the speed on 5 GHz over 2.4 GHz.
Dan
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee, just another user.