Tried to post an answer to a thread I found about this topic, but the thread appears to be "Locked".
SO... new thread for an old problem that many people have been fighting...
My wife and I both have Xfinity Mobile Galaxy Note 9 phones, but despite having assigned a nickname to each phone in the Xfinity Mobile app, hers kept appearing in my xFi Connected Devices list as "Galaxy Note 9", instead of the assigned nickname.
WHAT WORKED FOR ME: (Should work on any Android phone)
Cliff Notes Version for tech savvy folks: Renaming the phone in its Android Settings/ About Phone data section is the solution.
For folks who need a little more help, here are step by step instructions:
1. On the phone that you are unable to rename, Swipe down to show the Notifications screen. Go into the Settings (the Gear icon near the top right the screen), then scroll all the way to the bottom, to the About Phone section.
2. Tap on About Phone, and the Name of your phone is displayed above all the other cool info like Software Version, etc. Click on Edit just below the name of the phone, and rename it as you wish. Click Done.
3. Go back to the phone's Notifications screen, and turn off the wireless connection.
** NOTE ** (I did this next step because I assumed xFi would persist in recognizing the same device by its old name unless I wiped the slate clean by using the Forget Device command.)
Forgetting a Device will erase xFi's usage metrics and history for that device. If that's important to you, I suggest you try Steps 1-3, then skip to Step 5 and see if your method works.
If it does, you may not need to perform Step 6, either. Good luck)
4. On another phone or computer connected to the same network, open the xFi app, go to the Devices list, find the device name you want to change, and tap the right arrow (>) to open the device info screen, then scroll down to More Info/ Device Details,and tap the right arrow (>) to open the Device Details screen. Tap Forget Device at the bottom of the screen, and Forget Device again at the bottom to confirm your intention. xFi will then forget that device's properties. (xFi will only allow you to Forget a device while it's not connected, hence Step 3.)
5. On the renamed phone, turn the wireless connection back ON. It should automatically connect to the router.
6. xFi should show you a notification that a New Device has connected to your network. It will give you the option to Name the Device, then Personalize it by assigning it to a User's profile, etc. Go ahead and name the device and assign it to the appropriate User's profile, but if you didn't successfully rename the phone per Steps 1 and 2, you'll probably be swearing in a short while...)
7. Look at the Connected Devices list in xFi: with luck, you should see the device on the list with its new name. Note: I have observed some latency in data updates in xFi, so give it a few minutes to refresh its list data.