It's possible both phones have different SIM cards. A SIM card can malfunction or be incompatible with a specific device. Have you tried swapping the SIM cards between the two phones to see if the issue follows the SIM or stays with the iPhone?
Explore troubleshooting for iPhone: If swapping the SIM cards doesn't help, there might be an issue with the iPhone itself. Here are a few things you can try to get your iPhone's network working again:
Are the Service provider same for both SIMs? You have not mentioned that part.
- Simple fixes: Restart your iPhone, toggle Airplane mode on and off, and make sure Cellular Data is enabled in Settings.
- Network reset: Resetting your iPhone's network settings can sometimes resolve connectivity problems (Return iPhone settings to their defaults - Apple Support). This will erase your saved Wi-Fi passwords, so be sure you have them written down before proceeding.