I will try and be constructive as it seems some of the requested info is a secret.
Probably worth reading “how to get the best out of this forum” pinned to the top.
Did you know there are several Sim800 libraries? Should we guess?
I use the small red Sim800L with a 3.3v pro mini and it works fine. I power the Sim800L with a Li Ion 18650 battery. The Sim800L (red board) needs 3.4V-4.3V @ 2A (peak) Some can take 5V apparently but mine can’t.
The Sim800L logic level on this board is 2.8v so you may need a voltage divider to lower your controllers TX pin to a safer level. Again Some have not done this and have been ok, depends if you want it reliable I suppose.
To start with I would use a basic serial relay sketch to communicate with the Sim800L and send it AT commands to see whether you are able to send/receive an SMS.
When I started with the Sim800L I found this YouTube video very informative (it’s using the Sim900 module but will work with the Sim800L, has great info and program supplied)
Hope this helps.