Can't Split 2.4ghz and 5 ghz Connections

I'm very disappointed and am hoping that someone might have a workaround. I recently received a new router model from Xfinity, which is supposed to support faster speeds. Unfortunately, my smart light bulbs (all smart bulbs actually) only work on a 2.4ghz connection and the new router has combined the 2.4 and 5 into one connection. My understanding is that you should be able to split them on any router, but it's not working on my new router. I tried myself to split them in the Xfinity app and it appeared to work, but only one connection showed up under my devices' wifi connections list. An elevated Xfinity support tech (after regular tech had tried also) attempted to split it, but she said the system wouldn't let her. She basically told me it wasn't possible. Now I'm unable to use my smart bulb and had to buy a bluetooth-only bulb that doesn't work with Alexa. This is leaving a bad taste in my mouth. Any advice would be appreciated.