1. Are you using a power supply that was provided to you with the device?
2. Have you tried ANY other SIM cards?
3. What is the full name of the product, maybe the device that you bought isn't compatible with frequency bands that your UK operators use?
Navigate to this page: https://teltonika-networks.com/lt/product/rut950/ and go to Ordering, you should find product codes there, see if yours is compatible with your operator's frequency bands.
4. Maybe you have any other device from our production that you could test these SIMs on?
5. Check if you're not forcing particular network type and if your antennas are alright and screwed in tightly.
6. If nothing makes sense after all these options above - please send me your troubleshoot file via private message. Download it only after you tried inserting the SIM and waited for your router to connect to the network, so I could see the process in the logs.