someone has managed to hack in to my iphone 6s. how do i get them off without losing my data?

If your iOS device is not jailbroken it can not be hacked

if your provider is telling you your device IP address is from another device that's not how an IP address works. They would need to identify the Media Access Controller (MAC) address and verifiy Apple changed the series of their own on the first three octets of their own address. So who is telling you this?

if you think your Apple ID has been compromised see this link:

https://support.apple.com/en-us/HT204145

You may wish to change your Apple ID log/pass with a complex password you don’t use for anywhere else.

If you share the same or similar log/pass combo with another service (e.g. social media, bank, email, etc.) and that service is compromised then anyone with that information simply needs to try the same combo elsewhere

To change your Apple ID password see this link:

https://support.apple.com/en-ca/KM205079

To Change the Email associated with your Apple ID see this link:

https://support.apple.com/en-us/HT202667

What makes you think it was hacked? Did you allow someone to use your device while it was unlocked?

Restore the device to factory conditions and set up as new.

Have you talked to your carrier to find out where the other phone is being used and how it got access to your data?