Ooooh, this is actually fun. Your IP does not change, even when switching from LTE, 3G, 4G and 5G. There's an excellent post in the old Sprint forums where a company network engineer explains how it works. I can't find it though, but it's pretty wild.
In brief, it is because internet connectivity on most cellular networks goes through single gateways and devices are effectively "tunnelling" to those gateways. As long as the tunnel stays up and the mobile device is still identifiable on the RAN then the IP address assigned from the internet gateway doesn't need to change.
What I found especially interesting is how it keeps track of the device, especially when transferring from say a LTE capable station to a station that only supports 3G.
