the file transfer over and I get nothing but #N/A in all the cells
I just ran into this also with the latest LibreOffice (6.4.4.2). Out of curiosity I installed the latest version of OpenOffice (4.1.7) and the DDE links worked as expected when pasting them in. I guess newer isn’t always better.
I am working on the same problem. Think or Swim uses RTD format in live streaming. Libre(newer) must not. From what you are saying, the older version of Libre Calc, must be using a RTD format. Wonder why that was changed in 6.4.