Why is SMS to Xfinity Mobile not supported?

There are a few things worth trying to fix this:

First, SimpliSafe has a knowledge base entry for this very issue called, "Why can't I receive texts or emails for my SimpliSafe?" which I suggest you read if you haven't. I would also pay special attention to this section:

Some wireless carriers will by default disable your phone's ability to receive text messages sent by email. Verizon and AT&T users can enable email texts right on their online accounts. Other wireless providers require you to call in for customer support and request that you can receive SMS messages sent to your phone's SMS email address.

For Verizon:

1. Go to Your Account's Spam Controls and log into your account when prompted.

2. On the bottom of this screen, there is a button labeled "Block all text messages sent from email" that you'll just need to deselect.

3. That's it!

Second, this older post on these forums has the following suggestion from SimpliSafe staff:

Unfortunately, Xfinity Mobile doesn't have its own SMS -> Email gateway and generally uses Verizon's [phonenumber]@vtext.com. Many of our customers have had success using the PagePlus/StraightTalk option. Give that a try and let us know if it works for you.

Note: PagePlus/StraightTalk use the same SMS and MMS gateways that are listed for Xfinity Mobile which makes sense my this might work and is also worth a try.

Third, you may want to try enabling SMS notifications via the web app instead of the mobile app per this thread form a few years ago. Someone in the thread reported it worked for them.

Fourth, did you re-verify your phone number after you switched carriers? If not, this thread from about a year ago suggests it may help to re-verify your number.

Finally, where exactly is it failing? You can't enter your number? You can't verify your number? You can verify the number but you don't receive texts? GIve us some more details please.