I tried a few apps and liked the "tinyCam Monitor Pro" the best, or rather it was the only useful one I found.

I have installed the Hikvision firmware and use the cameras with my Synology Surveillance station. It works reliably but not very fast.

Hello,
Yes, the app is very rudimentary, you could almost say "old-fashioned". BUT it works very well and very quickly. The app displays the pictures very quickly, even when using several cameras, and that's what matters at the end of the day.
That's how I feel about it anyway.