Is it safe to use a found WiFi router with a SIM card?

I recently came across a portable WiFi router on the street in Denmark, and it still had a SIM card inside. I tried scanning the QR code to set it up, but it didn't work. Now I'm wondering about the safety of using this router. Can someone access my data or monitor my activity if I use it? If I factory reset the router and use my own SIM card, am I in the clear, or should I just toss it? Any advice would be really helpful!