It's possible that I'll suggest something you've done, since I don't know what you've tried before.
Here goes:
In Settings - General - Keyboards - All Keyboards, enable "Shortcuts".
In Settings - Safari - Autofill, make sure "Names and Passwords" is enabled.
If Safari is in Private Browsing mode, it won't fill in your information automatically. It looks like this:
View attachment 69175
If that's the case, tap the button at the top right, and then you'll find "Private". Tap it to disable Private Browsing.