Device needs 2.4 Ghz connection but gateway won't provide one

I bought some LED string lights that required their control box to make a 2.4 Ghz connection to the XB8 Gateway supplied by Xfinity.  It wouldn't connect, no matter what was done to get this connection.  I have other home automation equipment that also requires a 2.4 band connection, and that does work with my wifi network.  Notably, these devices were connected BEFORE Xfinity asked me to upgrade the gateway, so they were actually connected using another, older gateway.  They still work, but I can get NO OTHER device that needs a 2.4 connection to work with my present router.  I even bought a new smart plug that needs 2.4.  It wouldn't connect to this gateway either.  I have to think that there's a problem with the gateway equipment or its settings, but I spent 1.5 hours on a chat with your technician two days ago, and he tried everything he could think of to make the settings allow for a 2.4 signal.  He failed.  Here's what's been tried to make this work:  (1)  I have tried the so-called "walk away method", wherein you physically take your phone away from the router so that it drops any 5 Ghz connection and you are left with only the 2.4 signal;  (2)  I have gone into the admin tool and disabled the 5 & 6 bands, leaving only the 2.4 connection for the router to send out;  (3)  I've used the Xfinity phone app to split out the bands, creating a different network for the 2.4 and the other signals, and note that when the router was ONLY sending a 2.4 signal, and the phone settings for wifi were set to ONLY use that wifi, the phone consistently gave an error message saying "cannot detect the network"; (4)  the gateway has been restarted multiple times;  (5) the gateway has been unplugged to effect a pure restart;  (6)  the phone attempting the connection was restarted.  If you've got things set for the gateway to only push a 2.4 signal, and no iPhone (two different ones could not see that network), then the logical conclusion seems to be that the gateway is just not operating as a signal transmitter of a 2.4 Ghz signal.  Surely the newest and most modern gateway offered by Xfinity is capable of generating both a 2.4 and a 5 signal, but all indications are that mine cannot.  So I'm asking you, is this normal behavior for your gateway?  I don't think the maker of the LED lights had faulty equipment (it had to be returned to Amazon).  Do I need another gateway?