To debug the logic contained within a fetchable procedure you need to use the shlib command to load its symbolic information into the debugger. If you want to stop the debugger just before the invocation of MYFETCH, use the following command:
shlib MYFETCH.dll
As result of this a breakpoint will be set when MYFETCH.dll is loaded and you can then use the env command to switch to the appropriate source code display within MYFETCH.dll and set breakpoints where you think it is appropriate.
If you do not issue the shlib command, the execution will work as expected but you won't be able to debug MYFETCH. Additionally, if a trap occurs within MYFETCH.dll you will not see the source line where the trap occurred but will simply receive a message indicating a hex offset with MYFETCH.