Maybe that's a trivial question, but after you changed the APN settings, did you save the changes? Have you double checked whether all setting are correct?
Unbelievable... I won't bother flashing 1.7 then and just wait another month and a half to see if they can provide a decent software to this (just) decent hardware.
I had trouble downloading too, but finally I've succesfully downloaded it with Free Download Manager. Here is the direct link to the ROM file to add it into the download manager: http://cdnfiles.romjd.com/umi/1/20151104235302_90895.zip
I can confirm that this v1.7, posted above,
may be dirty flashed via TWRP, so every setting and data, that's been previously on the device,
will remain intact.
And... The issue with the swapped Left and Right headphone channels has NOT been fixed...