Tried different ways.
#include "filename.h" do not work at all.
Put filename.h and filename.cpp in different directories.
- put the same directory as the ino file.
- put the sub-directory (/filename/) to the ino file
- put in the directory (filename/) parallel to the directory of ino file.
always report not found the filename.h file.
Arduino IDE version is 1.8.13
Can latest Arduino IDE only use library files?