In my opinion, for a 3.7V LiSOCl2, with its rapid voltage drop when exhausted (per the battery datasheet), I’d skip the booster circuit. I’d surmise the additional run-time gained is going to be quite small. From the battery datasheet, the peak current is measured over 100mS with a 2 minute recovery. The modem is going to consume peak current for about 2.3mS every 4.6mS in worst case 2G GPRS modes (at max power and worst antenna matching). So, the average current is roughly half the peak. If the battery can maintain 1A continuous, that should be enough.
I took a squiz at the Electron doc/schematic. The ublox modem is normally powered via the PMIC output (3V8 line), and has a 200uF cap. close to its Vcc.
In this case, you will need to directly connect to the Li+/Gnd, or JST battery socket, as the PMIC expects a minimum of 3.9V on VIN. The PMIC won’t be doing much, but it won’t charge the battery as there is no VIN (as long as you don’t connect both battery and USB at the same time!). You can, as Rick K mentioned above, disable the PMIC charging functions to be completely safe.
So, this is (almost) a direct connection from battery to the modem and its existing 200uF ‘tank’ capacitor. I’d say that you would be fine with just that battery and a 330uF tantalum in parallel, the latter as close to the Electron’s Li+ and Gnd as possible.
Note that the PMIC can disconnect the battery when it drops below 3.0V (adjustable) - but this is also the minimum operating voltage of the modem, anyway.