Split 2.4 & 5 but my devices won’t connect

I have a new gateway. I split the 2.4 &  5 bands. I disabled the five in the six GHz and have a segregated 2.4 . Most of my devices will connect to the 2.4 but I have two that will not. How do I get these two to Connect? a technician said I need a wireless router, if I purchase that & I put the gateway in Bridge mode, do I have to reconnect every single device in my home? Why can’t these devises connect?  The upgraded gateway is causing more problems & costing more $$$