I want to recover my BTC wallet and I only have the mnemonic and the wallet address

Hello guys. Some years ago I made a BTC wallet on a website or so that I don't remember on desktop or maybe it was a browser extension (?).

Now I want to recover it and I have the 24 letter mnemonic (full and correct I'm sure) and its address (bc1....). I kept the mnemonic in my passport cover and the address I retrieved from my friend who has sent BTC to me multiple times.

Now I want to log in to it and wherever I go it asks me for "PRIVATE KEY".

How do I retrieve this private key thingy? At that moment the site/app just highlighted: write this down and preserve it. And I did just that. And here I am not being able to access my funds. :(