The best Primary Batteries that I've used is Energizer L91. They have a flat discharge curve, an extremely low self-discharge, can handle the 2 amp Modem requirements, and are fine in your -25 degree temperature enviroment.
Each will provide 3,500 mAH in this application.
You will see 1.6v / battery "in circuit" at 100%, down to 1.3v / battery reaching discharge.
That's a Voltage range of 4.8v to 3.9v using (3) AA L91 batteries in series.
You may be able to search and find reports for the Electron's Sleep Current on this forum.
If not, you can use something like
Then you parallel as many sets of (3) that you require for your mAH rating (3500 mAH each set).
These cells are $1.50 each.
As @ScruffR said, the self discharge profile for the battery will be a major factor. I haven't found a better primary battery without getting too exotic and expensive.