VLC lists RTSP as one of the stream types it can open on iOS.

It’s kinda tough overall to find free apps on iOS for self-hosted stuff since it costs the developer money to even have it listed.

Not strictly RSTP, but the “zmninja” app works with a Zoneminder server, which can record and manage RSTP/ONVIF cameras. I would really recommend some sort of NVR (like Zoneminder) if you have cameras since there’s a low probability you’ll be able to watch them live all the time.

Wouldn't it be easier to just view the cameras through the interface of your CCTV recording software? I personally use Shinobi for recording, and run Frigate for object detection and notifications through home assistant.

I got motioneye running on my server and the app on our phones

Restreamer is a great service to run on your machine and then load the feed on your iOS device. They have a decent UI for multi cameras, but then Frigate is just a few steps away…

I sort of gave up on this once I discovered Frigate. Now I just watch the streams from my browser. I bought a domain name and use Nginx Proxy Manager so I can access Frigate from anywhere.