PS4 lagging severely when other devices connect. Dual band modem - XFinity

#1
So we get 75mbps, I have my PS4 connected via ethernet cable. We have a dual band modem/router, which I know little about. I have my phone connected to 5ghz, PS4 was fine. Then my wife connected to wifi, and my PS4 started lagging to an unplayable degree. Neither of us were doing anything on our phones except instant messaging each other. I feel like my PS4 shouldn't be lagging that badly. Is it better to connect our phones to the 2.4 or the 5? Does it matter?
 
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