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Hi, I registered to this forum to help other people with my same problem with their TL-MR3420.

I spent more than a week of work before finding out it was a firmware issue.

Problem: When you try to access to Remote Management (or to reach a port that you have forwarded) you cannot if you are using your USB modem dongle, while it works if your TL-MR3420 is connected through WAN cable backup connection (blue port).
Solution: Upgrade firmware to a newer (beta?) version.

I have a European TP-Link TL-MR3420 V3 with original firmware: TL-MR3420(EU)_V3_160412

The valuable and professional USA TP-Link support sent me this firmware: mr3420v3_eu_3_16_9_up_boot(160517).zip

[EDIT]
I received a Database Error while uploading the attachment to this forum, so please tell me how I can send this attachment.
It is zip file, 3.69 MB (3.871.142 bytes),
CRC32: A50079DF
MD5: 805DF93134D5FE1C0F813F1C4C96EB6E
SHA-1: E3B8E6C12F4034CF727E2F468C0D78EE10611A53