You can refer to http://www.tp-link.com/en/article/?faqid=400 for troubleshooting

Hello to all..
Viva!

I had the same problem with my TL-MR3420 v.1 and with TL-MR3420 v.2

solved only for TL-MR3420 v.1:
download from TP-LINK site and downgrade the firmware to mr3420v1_en_3_12_8_up(110418).bin

Explanation:
what happens is that the new/latest firmware has a bug and only 4G modems get to work. The old 3G or 3.75G don't connect. It will show you the signal strength
100% and the modem works on any laptop/PC wit windows or Linux, but not with the TL-MR3420 v.1 that has the bin later than 110418 or any bin on TL-MR3420 v.2

So, for the new TL-MR3420 v.2 there is no bin file that can fix this. I have even tried to use the same mr3420v1_en_3_12_8_up(110418).bin, but of course no luck :-)

Actually I registered just to request TP-LINK to provide the solution for this problem BECAUSE I HAVE A BRAND NEW ROUTER that is not compatible with my modem that works
on a older router. I understand that an upgrade of a router should fix problems, not create new!
My MODEM is ZTE AC2746 3/3,75G CDMA2000

Best regards
Abraços

Victor Riquelme
+258 84 3373 160