Translated: How can I follow my partner's IP?
IP addresses are not assigned per individual. So if your partner switches from wifi to mobile on the same device, the IP will be different. Their IP could also be different if they travel to a different location even while on the same mobile connection.
If you're asking how do you follow what they're doing online, that's something you and your partner need to discuss as this is a private matter. Their privacy matters just as much as anyone else's. If you feel they're doing something wrong online, then you need to settle that with them directly.