Slow 2.4 GHz Speed, 5 GHz Working Fine

I have 60 mpbs plan through Xfinity, on 2.4 GHz on this modem I'm getting 25 tops mbps but a lot of times I'm getting 9 mbps or less. I ordered this off of Amazon - I sent the 1st one back bc I thought it must be hardware failure but the replacement is doing the exact same thing. I'm getting 54 mbps on 5 GHz and wired I'm actually getting above 60 mbps. This is getting very frustrating and I'm wondering if I should just send this one back as well and get another brand.... Is there anything I can do to get the 2.4 to work any better? My old router consistently got in the 40's.