You need to solve two problems here: battery charging and undervoltage lockout
The good news is that the voltage range of your parts (the Arduino's AVR microcontroller and the SIM7000 module) is wide enough that once we step the voltage down for your 18650, we do not need to mess with it any further -- they will run just as well on 4.2V from a fully charged cell as they will from the 3.something volts from an empty one. However, we will still want an undervoltage/brownout protector to keep the cell's protection circuit from having to kick in on a regular basis, and we will also need a suitable charge controller that can take car "12V" and charge the battery.
While Li-Ion charging facilities are common, most of them run off of 5V, not directly off of the variable "12V" automotive supply voltage, and so it goes for the SIM7000 module's charger, rendering it not useful for our application. As a result of this, we need to look further afield for a battery charger IC, and Linear Technology (now part of Analog Devices) comes through with the LTC4002-4.2.
At under $4 in unit quantities, it is not particularly expensive, and with the addition of a P-channel MOSFET, two Schottky diodes, and a handful of passives, it provides a complete step-down switching battery charging solution. It also comes in an easy-to-solder SO-8 package, and is well-documented, with detailed design procedures and simulation models (using LTspice, of course). Paired with a low-loss PMOS in SO-8 or IPAK, a couple of 20V/3A Schottky diodes, and a suitable inductor with at least 2A of current handling capability, it will be able to both charge the battery and handle the current draw spikes of the SIM7000 module.
This leaves undervoltage protection, and this is easy -- just about any reset supervisor IC with a threshold in the 3.3-3.6V range can be used, as holding both the AVR and the SIM7000 in reset will put them into a low-power-draw state. Optionally, the supervisor can also provide manual-reset and/or watchdog timer functionality as well, as the watchdog may need to reset both the AVR and the SIM7000 to clear a hung condition.