Can't connect to 5ghz band after setting up a new gateway

I upgraded my Internet plan and just finished installing the new gateway. During setup, I kept all my previous settings, including the split bands. Most of my devices are compatible with 5ghz except for a few smart devices that only work on 2.4ghz.

But now, for some reason, I can only connect to the 2.4ghz band. The Xfinity app says that the 5ghz band -- as well as a new 6ghz band -- is active and broadcasting, but I can't see or connect to it from any of my devices. I figured consolidating the bands back to one and then splitting them again would fix the problem, but I'm getting a generic "something went wrong" error when I try to make any changes to my wifi settings. I can't even change the SSID on the 2.4ghz band, which is the one that's working.

Xfinity Assistant says my Internet is fine, and that if I'm still having issues, the next step is to send a technician to my house. I don't want or need a technician to come to my house, I just need to change my settings. Is there a way to bypass the app's interface and change this directly? If not, can a tech support agent connect remotely and fix it from their end?