Since there have been lots of API changes in Unity between 5.5 and 2018.4, I would recommend upgrading to the package manager version of TextMesh Pro and following the upgrade procedure outlined in the following sticky thread at the top of this forum specific to this process.
As usual be sure to backup the project first and carefully follow the direction / outlined steps. Also be sure to upgrade to version 1.5.0-preview.8 which is available in the Package Manager but might require you enable in the Advanced tab of Package Manager to see preview packages.
Let me know if you run into any other issues in this process.