No lock at all is scary, do you have your email account on your phone as well? I’d be more worried about that. If you have Gmail you should use the “Manage devices” option and force the phone to sign out. Also consider changing you email password, and checking that no odd filters have been set up or anything.
As for Monzo, you can force the app to log out on your lost phone by logging in on another phone of the same type. So if an Android phone, use another Android phone to log it; in an Apple phone, use another Apple phone to log in. (You can log out immediately after if it’s a phone you have to hand back to someone else).