Hi,
Then I recommend:
A step down (buck) regulator for 7..12V input to 3.5...4.5V output and max 2.5A. No need to be isolated.
It should have a worst case input current of 1.8A (7V_in, 4.5V_out, 2.5A, 90% eff)
Thus you could use a 7..12V, 2A rated common power supply.
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To keep heating in the Arduino low, choose low input voltage --> maybe 7.5V (to have some headroom)
The same for the modem: 3.7 ... 4.0V.
To keep GND bounce low:
Wire the supply to the step down. And use the GND of the step down as star point.
Add a bulk capacitor as well as a fast ceramics capacitor at the star point.
Klaus