Andrew, you’ve done a lot of the heavy lifting, just a couple steps to go.
As you noted, your APP settings are correct. Exporting file declarations etc. That causes the generator to generate the EXP file. Looking in there it looks like your stuff is in there, so that’s all good.
The Linker uses the EXP file to create a LIB file. So the next step to check is a) your app includes that LIB file and b) the LIB file you are including is the one you think it is.
a) In the app with the error, you can go to the solution, to the project, to the Libraries section. You should see the name of your Data DLL Lib there. If it’s not, then it can be added to your app as a new External Module.
b) Unfortunately the default RED file in clarion separates the DLL file from the LIB file - and specifically separates the LIB into a debug or release folder. This is the root of many Linking errors. See Editing Default Redirection RED file to create .LIB with .DLL in project folder to correct this.