What are the risks having visited a counterfeit or phishing web page on iOS?
It's very unlikely that you were infected with anything or that anything negative happened overall if you did not enter your credentials. My reasoning for this is as follows:

It asked for your credentials, which indicates the goal was phishing not infecting your devices
Drive-by infections are not impossible, but they're also not easy. Unless you're a high profile target it's unlikely you would ever interact with one.

I don't believe there is any reason to be concerned based on how you described this.