Your developers chose a really annoying UI framework to work with. There's 2 things going on, first the Window is inside an iframe, and 2nd it's very slow loading and appears to create all the elements first but hidden, then shows them later.
So from the testing side, since all these steps are not synchronous. You have to do a bunch of waits. After switching to the window, notice I have to first wait for the element to be rendered inside the iframe, then I have to wait for the element to be shown in order to click it.
Here's how you would do it in Python
from selenium import webdriver
from selenium.webdriver.support.wait import WebDriverWait
driver = webdriver.Firefox()
learn_more = WebDriverWait(driver, 60)\
.until(lambda driver: driver.find_element_by_css_selector("a[href='/platform'].Btn"))
#wait until element visible
#verify banner has expected text.
banner = driver.find_element_by_css_selector(".Section-banner h1")
self.assertEqual("The Telerik Platform", banner.text)