https://flows.nodered.org/node/node-red-contrib-webhookrelay

https://community.home-assistant.io/t/problem-with-mqtt-retain/30158

https://www.hivemq.com/blog/mqtt-essentials-part-8-retained-messages/

https://forum.arduino.cc/t/example-of-interrupt-based-version-of-reading-serial-port/79735

int16_t wait(char* Values, uint16_t timeout) {

uint16_t Length = strlen(Values);

char InputBuffer[Length + 1];
strcpy(InputBuffer, Values);
char CompareBuffer[Length + 1];
memset(CompareBuffer,0,sizeof(CompareBuffer));

uint16_t Quantity = 1;

for (int16_t n = 0; n < Length; n++) {
if (InputBuffer[n] == ‘,’)
Quantity++;
}

char* InputTokens[Quantity];
memset(InputTokens, 0, sizeof(InputTokens));
char* CompareTokens[Quantity];
memset(CompareTokens, 0, sizeof(CompareTokens));

InputTokens[0] = InputBuffer;
CompareTokens[0] = CompareBuffer;

uint16_t TokenPosition = 1;
for (int16_t n = 0; n < Length; n++) {
if (InputBuffer[n] == ‘,’) {
InputBuffer[n] = 0;
InputTokens[TokenPosition] = &InputBuffer[n + 1];
CompareTokens[TokenPosition] = &CompareBuffer[n + 1];
TokenPosition++;
}
}

uint64_t timer = millis();
char c;

while (millis() – timer < timeout) {
while (mySerial.available()) {
c = mySerial.read();
Serial.print(c);

for (int16_t n = 0; n < Quantity; n++) {
Length = strlen(CompareTokens[n]);
if (c == InputTokens[n][Length])
CompareTokens[n][Length] = c;
else
memset(CompareTokens[n], 0, Length);

if (!strcmp(InputTokens[n],CompareTokens[n]))
return n;
}
}
}
return -1;
}

Source: Waiting for AT COMMAND correct response – Using Arduino / Programming Questions – Arduino Forum

The above is an example of Weixue. In my case, the second and fourth items need to be changed
AT+CMQCON=0,3,”myclient”,6000,0,0,”admin”,”password” You need to add admin and password. If you don’t add it, you cannot connect.

Source: SIM7020C NB-IoT MQTT communication Raspberry Pi 4B – Programmer Sought