How do I force some devices to connect to 5GHz?

Are they even 5 GHz capable in the first place ?

Assuming your devices support 5 GHz, one way you can force connection to 5 GHz is to separate the 2.4 GHz and 5 GHz signals. You can do that in xFi. Then you can connect to whichever one you want.

I know Xfinity doesn't recommend it (for whatever reason), but I've done it and haven't had any issues.

Hi @paddockmoore! Thanks for posting to the Xfinity Forum. What type of devices are you trying to connect? @g-hodgson is correct. Although we support the integration of third-party devices with Xfinity Home and Xfinity xFi, Comcast does not provide support for third-party devices. If the device(s) you want to connect is compatible, you can connect and control your devices using the xFi or Xfinity Home Security App. For more assistance, check out these links: 

* What Is Works with Xfinity?

* Works with Xfinity Compatible Devices for xFi 

* Linking Works with Xfinity Devices to Xfinity xFi 

* Controlling Works with Xfinity Devices in xFi

* Some devices work with the Xfinity Home Security System and can be found here, Controlling Works with Xfinity Devices in Xfinity Home. 

I hope this helps.