Since it's allegedly your backup, you should at least know a couple of possible passwords or parts of it for bruteforce method to be viable, yes?
If so, try BTCRecover and construct a "token file" based from the possible password parts that you can remember.

If you need assistance on it, you must describe it in detail without being specific on the password itself,
like for example: 8 to 10 characters, first 7 letters could be word1, word2 or word3, followed by numbers then a symbol.
Since it's a backup, if you've changed the password after that backup is created, you should use the old password.

If you prefer DYOR, here's its documentation: https://btcrecover.readthedocs.io/en/latest/

Otherwise, there's no way to retrieve your bitcoins since your private keys aren't saved anywhere but your wallet.dat file and its backup.