TypeError: elem[prop] is not a function

E2E testing in webdriveio. I want to click a button inside an iframe.

let iframe = browser.$('#fullmessage')
        browser.pause(1000)
        browser.switchToFrame(iframe)
        browser.setTimeout({ implicit: 10000 })
        let clickAgree = $('a[class="button is-success"]')
        clickAgree.click()
        browser.switchToParentFrame()
        browser.pause(3000)