MPPT (Maximum Power-Point Tracking) is a process that provides the maximum-possible energy transfer from the PV panel to the battery. (See Maximum power point tracking - Wikipedia) You can think of MPPT as being an impedance-matching process. MPPT adjusts the load to match the output impedance of the PV panel. The output impedance of a PV panel is higher at low irradiance than at high irradiance, so the load impedance is increased at low irradiance by MPPT to compensate.
The CN3165 charge-control chip resident on the DFR0559 provides quasi-MPPT tracking that works best around 25°C with the prescribed Wisblock PV panels (e.g., HX-140x90). The MPPT in the CN3165 uses a “constant-voltage” algorithm, which takes advantage of the fact that the max-power voltage (Vmp) is roughly constant as a function of irradiance. So, with constant-voltage (of about 5V in this case), the load draws as much power as possible from the PV panel over a wide range of irradiances.
However, the constant-voltage algorithm does not compensate, in particular, for changes in the PV-panel temperature. Vmp decreases with increasing temperature, which constant-voltage will not take into account. Note that PV-panel temperature likely increases with increasing irradiance (i.e., the PV panel gets hotter when the sun is brighter), meaning that the constant-voltage MPPT really doesn’t completely compensate for changes in irradiance. The nominal expected voltage of the HX-140x90 at 25°C is 5V, which is the voltage that the DFR0559 MPPT expects.
Note that improved energy capture can provide faster (and more reliable) recovery from bad operating conditions, like blanketing snow. Also note that having over-sized energy management at the outset is advantageous, since silicon PV-panel power and lithium battery capacity are likely to degrade over time. And you might also encounter extended adverse weather conditions.
Using the solar array I have installed on the roof of my house as a reference, I estimate that the HX-140x90 should be able to make 8 Wh on a sunny day, and maybe 4 Wh on a cloudy day. The RAK4361 will need about 2 Wh per day. The HX-140x90 really should be sufficient. A fully-charged 4200 mAh battery should be able to supply power to the RAK4361 for more than a week.