I beg to differ - the IP address for a mobile phone is not "the same". Unlike static IP addresses from a physical location (e.g. when using a desktop computer at home or work), IP addresses on mobile phones are dynamically assigned, meaning that they constantly change, sometimes within minutes. That's because mobile phones connect to cellular towers, which have a massive pool of IP addresses from where IP addresses are assigned to mobile phones as they connect in & out of the towers. A good way to test it, use an ip checker website (just google one on your phone), then either turn your airplane mode on/off or move between areas and you'll see that the IP changes.
So IP will not be an issue with a phone. But Instagram collects other "fingerprint" / device tracking info (nobody knows exactly what) and that can potentially be used to allow/disallow the creation of a new account.
The low cost option - start with erasing all Meta apps (IG, FB) and related data on your phone. Then try again.
The pricier option - if the above fails, sign up to an anti-detect phone or anti-detect browser service and use that setup. There is plenty of info on those types tools and setups here on BHW (use the search function above to look it up).