well,
ok i can share about my experience on TP-Link TL-MR3420.

MR 3420 although setting done at Mac address...Static IP...DMZ ...virtual server is confirmed still not working.
Port forwarding practically cannot work
Mine is version v2

But i believe your Huawei E3256 will be able to do "port forwarding automatically" itself if dialing in conjuction with MDMA.

I have problem connecting to GameRanger initially.
Using the default Mobile Partner menu,
i can connect to internet and surf but cannot play online game at GameRangers as it always show "NAT Error".

Which is why i bought the MR 3420 expecting it to work.
Well, it failed miserbly despite using Huawei E398 4G LTE

Discover that creating a Dial-Up name 'MDMA" and using it together with MDMA software.
I am able to play without problem at GameRange.

Now my MR 3420 is a white elephant..Damn TP- Link