Nothing I do will enable the submit button? No errors are thrown in the console either, but I am really struggling to get this last bit completed!

const checkBox = document.getElementById("accept");
const submitBtn = document.getElementById("submitButton");

checkBox.addEventListener("click", function() {
    let valid = false;
    let isFormValid = false;
    if (checkFirstName() == valid &&
        checkLastName() == valid &&
        checkEmail() == valid &&
        checkTelephone() == valid) {
        isFormValid = true;
    }
    if (checkBox.checked && isFormValid) {
        submitBtn.disabled == false;
    } else {
        submitBtn.disabled == true;
    }
});