iPhone 13 no notification when lock screen

So I recently got an iPhone 13 mini and kept my old iPhone 8. Both now have separate phone numbers and SIM cards but have the same iCloud. The iPhone 13 works fine accept when I’m using my iPhone 8. (Eg iPhone 13 might be charging and I might be scrolling through Facebook or on you tube on my iPhone 8). When my iPhone 13 is on Lock Screen and I receive a text message to iPhone 13, it does not wake up the phone. WhatsApp does, but text messages do not. So I could be watching a 30min you tube video and be unaware that I have 10 text messages, even when my phone is charging right next to me. It still rings for phone calls and wakes up for other notifications, it’s just text messages it’s having issues with. When my old iPhone is not in another room, the iPhone 13 works as normal. Ps I don’t have an Apple watch or anything else connected to it.