FileMaker 11 has the old file-format (.fp7), therefore no chance to view a FM13 file with FM11. FM12/13-format is .fmp12
"In addition, FileMaker Server 12 and later will only host files converted to the .fmp12 format."
So the issue is that, I assume, the files being hosted are in fmp7 format and not fmp12. No matter which version of the FMP client you use, FMS13 will not serve up files in the old format. And of course, if you somehow convert the hosted files to fmp12 format, Filemaker 11 client will not read them.