IS it best to separate the 2.4 ghz and 5 ghz wifi channels?

I would also use the hybrid 2.4 and 5 GHz network which auto-selects the band and channel for each device as the primary network.  I don't know if there is a proper name for it; I call it the "default" network because that is how the gateways are configured out of the box.  A separate 2.4 GHz network, with lesser security if needed, can be added for older devices or ones that won't connect to the default network.

Before I had the T-Mobile gateway, I used a Netgear Nighthawk 802.11ac router.  It had both 2.4 GHz and 5 GHz bands.  I set up connections for my TV to both.  Occasionally (maybe once a month), the TV would lose its connection to one and I'd have to go into its settings to switch to the other.  I have not had to do that in the year that I have had it connected to the default network on the T-Mobile gateway.

I have not had any trouble with any of my devices in my home, except for the ancient desktop computer and printer which would not work with the WPA2/WPA3 + AES security on the default network; and they are so old, they don't recognize 5 GHz signals either.  I made a 2.4 GHz WPA/WPA2 + TKIP/AES network just for them.

I have not seen a reason to add a dedicated 5 GHz network in my home.