You need to connect the modem directly to the WAN port on the back of the MR3420. Because its a 3/4G router, you need to login to the MR3420 and configure 'WAN only' (or 'WAN preferred' if you are...

Hello I have been using a beta update for the MR200, which you should find is available through tp link support. It did fix the problems I was having with l2tp and ipsec.