Before we buy a modem, we need to go to tp-link official website to check which modem is compatible with our 3G/4G router:
http://www.tp-link.com/en/products/?categoryid=202&mode=complist

Here I can't find Sweden, which means tp-link's 3G/4G router now doesn't have a compatibility list for Sweden :(
So it's quite understandable that E3131 can't work with E3131.

But you can try the guide in this FAQ to have a try: http://www.tp-link.com/en/article/?faqid=400
Maybe it will work, good luck :)