AT&T U-Verse - WiFi question - Devices can't talk between 2.4ghz and 5ghz

Hi- Regular U-Verse customer, just updated my router from the 589 to 510. For some reason, my wifi devices that are on different networks (printer on 2.4) and laptops (on 5) can no longer talk to each other. How is this possible? Am I missing something stupid here? thanks in advance DG