The modular sensors library that Beth references depends on TinyGSM for the majority of the cellular connection process. You may want to look at the code and examples in that library for help communicating with the GPRSBee (which is a SIM800 chip).
Unfortunately, I think if you’re successfully querying for csq and only being a response of 0, that probably means you just don’t have any signal. Having amazing 3G or 4G doesn’t mean there’s any 2G at all. Beth has found some quirks to the sequence of connecting the GPRSBee to that could also cause it to falsely report 0 signal. From the ReadMe for modular sensors:
Note for GPRSBee modems: To start the modem you will need to power the logger board off, connect the battery to the logger board, and finally attach the modem to the logger board. Then you may power the board and run your sketch. We have found that attaching a GPRSBee modem to power in a different sequence results in the modem reporting zero signal strength. Note, the Mayfly connected to a computer via USB does not supply sufficient power to the GPRSBee. If the community finds this true for other modems, please let us know.