Hello Mcaton47,
This sounds like you may be using "Sign in with Apple" for the Accessory App on your iPhone. If so, this will not allow you to sign-in on their website (using the subscription) since their website does not offer a "Sign in with Apple" button.
How to use Sign in with Apple - Apple Support
"If you don't see a Sign in With Apple button on an app or website, it doesn't support Sign in With Apple yet."
Looking on https://support.ancestry.com/s/ they seem to only address PayPal subscription issues of this nature. You may be able to update your username and password in the App to use on their website. https://support.ancestry.com/s/article/Changing-Your-Password-or-Username-in-the-Ancestry-App
Some other membership issues seem to recommend contacting Ancestry support so they can help locate the subscription which, may be the best option for you. https://support.ancestry.com/s/contactsupport
I hope this helps.