To avoid having to type the absolute path to a command, shells introduced the $PATH environment variable, each directory is separated by a : and searches are done from left to right. cron often clears the whole environment, including this $PATH variable. Therefore, the script may behave differently in your cron compared to the behavior in the shell.
Use absolute paths
Run which db2 as db2inst1 user to get the full path for the db2 program. If the output is /usr/bin/db2, your cron command would look like:
/usr/bin/db2 connect to myDB2
Set the PATH variable
Run echo "$PATH" as db2inst1 user to get the $PATH variable and make sure this variable is available in your cron script too. For example, if the output was /usr/local/bin:/usr/bin:/bin, you would put the next line in the top of your shell script:
export PATH="/usr/local/bin:/usr/bin:/bin"