For reasons like this, I use different SSIDs on 2.4 and 5GHz. I haven't had any issues with any Roku connecting, but I've always wanted to ensure my device (whatever it is) is connected to the band I prefer.
We have the eero 6 mesh also. I tried the temporary pause of the 5 GHz which for the Roku TCL TV, never did connect. However, when I established a "guest" on the network and paused 5.0, signed the Roku TCL TV in with the "guest" password, as the only device, it grabbed the 2.4 GHz and has remained connected. However, I wonder if/when there is a Roku update or there is an internet outage, if it will remain. I am assuming it will. We also had two other Roku devices independently connected to smart TVs. They never have worked even with my troubleshooting and "fix" for the Roku TV, so I'm just donating them and obtained a competitor brand plug-in. As rhfactor theorizes above, I believe in our system, it is Roku technology cannot capture the 2.4 GHz in a mesh system.
Any Roku device that is connected to WiFi will reconnect to the same exact access point after a power cycle. Even if it has credentials stored for a different SSID, it will still only reconnect automatically to the same SSID/password. I don't know if a mesh network confuses anything for Roku devices. I have both mesh and "normal" access points in my home, and they all seem to reconnect correctly to the most recently connected AP.
