ipad pro cellular: could not activate mobile network

background

• i have been using my ipad pro for about a week, without issues.
• since yesterday, i get the 'could not activate mobile network' popup.
• it has been acting weird in other situations (often stuck, apps crash more, etc.) since a day or two as well, but that isn't necessarily related.
• i am from belgium, so info about us carriers won't help me

things i have tried

• a new sim card
• restore network settings
• restore completely

nothing seems to work. does anyone have an idea of what could be wrong? (don't tell me my cellular hardware is broken)