How to find previous IP address?

Hi all, I have a super urgent question that would really need some help. My iphone 11 has updated the system a few days back to the newest one, however, I notice that after the update, the system automatically was reassigned with a different IP address from the previous one. So even though I am using the same device, the device has a different IP address even if I am using the same route at home. Everything is fine until I forgot the password to an APP that only send the password reset instruction if the request is sent from the same device with the same IP address.

Really appreciate if anyone can let me know how I can find the old IP address my iphone used so that I can set a static IP address on my current phone to retrieve the forgotten password.

Thank you a lot!!!