Did you use the latest firmware of TD-W8970_V1_130828? If not, you can try to upgrade the firmware. dan1el wrote I am having the exact same use model (with 3g modem) and facing the same issue...

The E5331s-2 and zte mf70 can share the internet theirselves, right? If so, there is no necessary to make it work with a 3G router. The E5331s-2 and zte mf70 are the combination of 3g modem and 3g...

Now a new fimware has been published, maybe it can solve your issue. http://www.tp-link.com/en/support/download/?model=TD-W8970&version=V1#tbl_j

8951 in bridge mode ,router connect with it,dial up if you can get the Internet access,there is no problem