Sorry, I am not entirely certain of the missing Instagram Icon, I am seeing social icon buttons towards the bottom of your post and on the left-side and both have the Instagram Icon. Please clarify.
You can create this as a Global block using the new feature Global Blocks (X > Global Blocks), then copy the shortcode of your block and paste it to the bottom of your posts. Or add it using the Global Block element if your post is built in Cornerstone/Content builder.
I was able to open your pages in Cornerstone, this can be a caching related on your end, please clear your browser's cache or try using the incognito/private mode of your browser.
Let us know how it goes,