Hi tsairidis xronis,
Thank you for reaching Microsoft Community. We're sorry for the trouble this has caused you. Allow me to ask few questions to help us further isolate the issue.
May I know which provider are you referring to?
Did it work before or it's the first setting up your SIM with this provider?
When did this issue started to occur?
What build version of Windows are you on?
For the meantime, you may try the following steps below to troubleshoot your device.
Force the device to shut down and restart.
Press and hold down the power button until your Surface shuts down and restarts and you see the Windows logo screen (about 20 seconds), then release the power button.
Run Surface Diagnostic Toolkit and check Windows updates including optional updates if available.
See: Fix common Surface problems using the Surface Diagnostic Toolkit (microsoft.com)
To check for updates: Go to Settings > Update & Security > Windows Update > Check for Updates.
Then try the solutions provided from this article, Can’t connect to cellular data on Surface? - Microsoft Support.
Let us know if none of the solution works so we can further assist.
Kind regards,
Lavenia