You must also add the rescap namespace IgnorableNamespaces in the Package.appxmanifest file as shown below.
<Package
xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10"
xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest"
xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10"
xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities"
IgnorableNamespaces="uap mp rescap">
<Capabilities>
<Capability Name="internetClient" />
<rescap:Capability Name="cellularMessaging" />
</Capabilities>
</Package>
When adding capabilities, keep them in the following order: 'Capability', 'rescap:Capability', then 'DeviceCapability', as there seems to be a problem when mixing them. (source)
Note that the app certification test will probably continue to fail. You need to request permission to actually use these capabilities.
Special and restricted capabilities are intended for very specific
scenarios. The use of these capabilities is highly restricted and
subject to additional Store onboarding policy and review. Follow the
steps below to request access to a restricted capability before
submitting your app to the store.
- Determine if you are eligible to submit your app to the store with a
specific restricted capability by looking at the table below. If you
are not eligible, any requests you make will be denied.
- If you are eligible visit the Submitting an App support page.
- Set the problem type to App submission and certification and the category type to Submitting an app using a restricted capability.
- Include the capability you are requesting access to and include a reason for your request. If you do not provide all the information necessary, your
request will be denied. You may also be asked to provide more
information.
Source