I have the SIM800 GSM addon hat for a Raspberry Pi 3 and got it to work with T-Mobile
Using this website as a reference:
I followed these steps:
sudo apt-get update
sudo apt-get install ppp screen elinks
Add the following to the file
#t-mobile is the apn
According to RPi 4 overlays readme you have to turn off Bluetooth to use UART0. You should test alternative UART pins.
Info: Disable onboard Bluetooth on Pi 3B, 3B+, 3A+, 4B and Zero W, restoring UART0/ttyAMA0 over GPIOs 14 & 15.
N.B. To disable the systemd service that initialises the modem so it doesn't use the UART, ...
I'm using SC16IS752 IC which is SPI to 2xUART converter. It works well with Raspbian Stretch.
It is a bit more expensive than FTDI chip, but there are two uarts and I don't have to use precious USB port.
How can Rpi4B python UART talk to XY PWM Signal Generators?
(1) Introducing XY-LPWM, UART support version of XY-PWM
(2) Using Win10 RealTerm terminal emulator to serial/UART talk to
(3) Using Rpi python program to serial/UART talk to XY-LPWM
(2) Using Win10 RealTerm terminal emulator to serial/UART talk ...