Weird Serial Output with SIM800L Module


I am working on an IoT-Project in my university.

I need to get the GSM-Module (SIM800L) to work (with my Arduino Nano).
First I found this Topic and it was almost exactly what I needed. Then I had some problems compiling it, because of the “SIM_SERIAL”-Problem that was discussed here.

Now it writes “þ‹ÿüþýþüüüÿüÿüüýýüþüÿÿüýþþýýþüþ” in the Serial Monitor and I don’t know why.

(I don’t know how to do these mockups for cable management, but I’ll try my best to show you how my cables go)

D7 <-> RXD (SIM800L)
D8 <-> TXD (SIM800L)
5V <-> IN+ (LM2596)
GND <-> IN- (LM2596)

LM2596 (5V to 4V):
IN+ <-> 5V (Arduino)
IN- <-> GND (Arduino)
OUT+ <-> VCC (SIM800L) (4V)
OUT- <-> GND (SIM800L)

VCC <-> OUT+ (LM2596)
RXD <-> D7 (Arduino)
TXD <-> D8 (Arduino)
GND <-> OUT- (LM2596)

I only added these two lines to the code of the first link

#include <SoftwareSerial.h>
SoftwareSerial mySerial(7, 8); // RX, TX

Another test project answers to the “AT”-Command with “OK” and only other commands answer with “ERROR”. So I guess my cables are correct and the Voltage is correct aswell?
I read something about connecting the grounds, but this didn’t work either.

What could be the problem? :confused: