If the user uses Access Point mode, the THETA is always at 192.168.1.1.
If you use Client Mode, you must use discovery.
Tip: Developing THETA "Client Mode" Applications
There are many examples for IP discovery. You will have to implement it for your mobile app.
I have not tried it on iOS, but these docs may help.
Implementing Bonjour across iOS and Android – Jaanus
Intro to Bonjour in Apple devices - Apple Support
From the first article:
(I know that technically it’s “Bonjour” only on Apple platforms. The Android implementation officially calls itself Network Service Discovery. And both of them are implementations of “zero-configuration networking” using “DNS-SD (Service Discovery)” and “Multicast DNS” and… enough alphabet soup yet? Let’s just use “Bonjour” in this post.)
this is a quick example with Python
Ricoh THETA V client mode WiFi discovery for use with companion script to run WiFi API to control camera · GitHub
Testing from Mac command line
dns-sd -B _osc._tcp