Do you use separate 2.4ghz and 5ghz ssid's and connect devices to them manually, or just let the gateway figure out which band is best?

I just received my TMHI 5g box yesterday (Sagemcom Fast 5688W) and I think my Roku TV's are connecting to the 2.4 ghz band because the speed test is pretty slow on those devices. Right now I have my gateway on default so the 2.4 and 5 are together and it supposedly picks the best one for you.

BTW is the Sagemcom Fast 5688W considered a good gateway unit? Is that what everyone is getting assuming they aren't getting the white, mesh capable unit?